- Array
- Greedy
- Tree
- Tree Relevant
- Math
- String
- Hash Table
- Bit Manipulation
- Stack
- Queue
- Heap
- Linked List
- Two Pointer
- Sort
- Recursion
- Binary Search
- Binary Search Tree
- Depth First Search
- Backtracking
- Dynamic Programming
- Regular Expression Summary
- Sliding Window Summary
- Graph
- Design
- Bash
🔍 good algorithm
✏️ smart code design
🎅: good question
❌: not good designed question
几个单独算法:
- Trie
- Union Find
| Title | Time | Space | Difficulty | Algorithm Note |
|---|---|---|---|---|
| 102. Binary Tree Level Order Traversal | O(n) | O(n) | Medium | |
| Title | Time | Space | Difficulty | Algorithm Note |
|---|---|---|---|---|
| 015. 3 Sum | O(n^2) | O(1) | Medium | 🔍问题关键是sort + skip duplicate |
| 016. 3 Sum Closest | O(n^2) | O(1) | Medium | 🔍sort + two pointer，根据three sum 和sorted list移动两个pointers |
| 018. 4 Sum | O(n^3) | O(1) | Medium | 🔍sort + two pointer，思路和015. 3 Sum 一样 |
| 026. Remove Duplicates from Sorted Array | O(n) | O(1) | Easy | Two pointer |
| 027. Remove Element | O(n) | O(1) | Easy | Two pointer |
| 031. Next Permutation | O(n) | O(1) | Medium | 跟556. Next Greater Element III 思路类似, C++可以用is_sorted_until + upper_bound() |
| 041. First Missing Positive | O(n) | O(1) | Hard | 🔍先置换, 把每个元素放在合适位置，再看A[i] == i+1 ? 不等于 return i+1, 最后如果还没return, return size +1 |
| 048. Rotate Image | O(n^2) | O(1) | Medium | 🔍
|
| 054. Spiral Matrix | O(m*n) | O(1) | Medium | 🔍定义 up, down, left, right 四个边界，每次loop 在最外围的一圈 |
| 059. Spiral Matrix II | O(n^2) | O(1) | Medium | 🔍思路跟048. Rotate Image 和 054. Spiral Matrix 类似 |
| 066. Plus One | O(n) | O(1) | Easy | |
| 073. Set Matrix Zeroes | O(m*n) | O(1) | Medium | 🔍two pass:1. 把如果matrix[i][j] == 0, 把matrix[i][0] 和matrix[0][j] 设为0, 如果第一列设0之前，有数为0，设col0 = 0。 2.从下往上loop, 碰到matrix[i][0]] or matrix[0][j] 为0, matrix[i][j] = 0, if col0 == 0, matrix[i][0] = 0 |
| 080. Remove Duplicates from Sorted Array II | O(n) | O(1) | Medium | |
| 118. Pascal's Triangle | O(n^2) | O(1) | Easy | |
| 119. Pascal's Triangle II | O(n^2) | O(1) | Easy | Easy DP |
| 121. Best Time to Buy and Sell Stock | O(n) | O(1) | Easy | |
| 128. Longest Consecutive Sequence | O(n) | O(n) | Hard | 🔍
|
| 169. Majority Element | O(n) | O(1) | Easy | |
| 189. Rotate Array | O(n) | O(1) | Easy | |
| 209. Minimum Size Subarray Sum | O(n) | O(1) | Medium | 🔍
|
| 215. Kth Largest Element in an Array | O(n) ~ O(n^2) | O(1) | Medium | 🔍
|
| 228. Summary Ranges | O(n) | O(1) | Medium | |
| 229. Majority Element II | O(n) | O(1) | Medium | 🔍Boyer-Moore Majority Vote algorithm |
| 238. Product of Array Except Self | O(n) | O(1) | Medium | 🔍res[i]表示 nums[0: i-1]的乘积，right 记录从结尾到nums[i+1: end]的乘积. 最后res[i] = res[i] * right; 也可以用left, right One Pass |
| 240. Search a 2D Matrix II | O(n+m) | O(1) | Medium | 🔍sorted matrix题目的关键是从第一行最后一个开始，如果当前数比想要的大, --col, 如果当前数比想要的小，++row |
| 289. Game of Life | O(m * n) | O(1) | Medium | 🔍跟238. Product of Array Except Self有一点点类似，先变化matrix到想要的格式, 然后再做transform到结果: 把下一代活的 |
| 334. Increasing Triplet Subsequence | O(n) | O(1) | Medium | 🔍用两个数表示b, c, c表示当前最小, b表示当前第二小, 当i都大于这两个数，return true, 不用担心i只更新更新c, 因为答案必须要迈过b |
| 384. Shuffle an Array | O(n) | O(n) | Medium | C++ srand(time(NULL)); rand(); uniform_int_distribution |
| 396. Rotate Function | O(n) | O(1) | Medium | 🔍mathematical induction F(k) = F(k-1) + sum - nBk[-k] |
| 412. Fizz Buzz | O(n) | O(1) | Easy | |
| 414. Third Maximum Number | O(n) | O(1) | Easy | |
| 419. Battleships in a Board | O(n*m) | O(1) | Medium | 🔍看源头，if [i][j] = 'X' 且 [i-1][j] 和 [i][j-1] 如果都不是X，count += 1 |
| 442. Find All Duplicates in an Array | O(n) | O(1) | Medium |
|
| 448. Find All Numbers Disappeared in an Array | O(n) | O(1) | Medium | 思路与442. Find All Duplicates in an Array一模一样，两种方法也一样 |
| 565. Array Nesting | O(n) | O(1) | Medium | DFS, 把visit的点变为-1, nest array是循环，所以起点无论是哪个点进入都可以得到完整的循环, 比如 a->b->c->d->a 不会有 a->b->c->d->b |
| 566. Reshape the Matrix | O(m*n) | O(1) | Easy | |
| 581. Shortest Unsorted Continuous Subarray | O(n) | O(1) | Easy | 🔍
|
| 605. Can Place Flowers | O(n) | O(1) | Easy | |
| 643. Maximum Average Subarray I | O(n) | O(1) | Easy | 最简单的sliding window |
| 661. Image Smoother | O(n) | O(1) | Easy | 跟289. Game of Life思路一样， 一点不一样的是把下一代的数右移8个bit, 之后再第二次pass matrix, 每个点>>8 左移8个bits |
| 665. Non-decreasing Array | O(n) | O(1) | Easy | 🔍两种operation: 1.nums[i-1] = nums[i] (降), nums[i] = nums[i-1] (升), 降优于升 |
| 667. Beautiful Arrangement II | O(n) | O(1) | Meidum | 🔍brainstorm |
| 670. Maximum Swap | O(n) | O(1) | Medium | 🔍
|
| 674. Longest Continuous Increasing Subsequence | O(n) | O(1) | Easy | |
| 697. Degree of an Array | O(n) | O(n) | Easy | |
| 713. Subarray Product Less Than K | O(n) | O(1) | Medium | 🔍😍 Sliding Window |
| 845. Longest Mountain in Array | O(n) | O(1) | Medium | 🐸 |
| 918. Maximum Sum Circular Subarray | O(n) | O(1) | Medium | 🎅🎅 Kadane's algorithm |
| 997. Find the Town Judge | O(n+t) | O(n) | Easy | 🎅 In-degree, out-degree |
| 1375. Bulb Switcher III | O(n) | O(1) | Medium | |
| 1380. Lucky Numbers in a Matrix | O(m*n) | O(m+n) | Easy | zip(*m)获得column in list, set intersection |
| 1389. Create Target Array in the Given Order | O(n^2) | O(1) | Easy | ❌ |
| 1394. Find Lucky Integer in an Array | O(n) | O(n) | Easy | :pencil2: Loop C++ Map Key Value |
//Inorder Traversal
//stack,
class Solution {
public:
vector<int> inorderTraversal(TreeNode* root) {
vector<int>res;
stack<TreeNode*>stk;
while(root || !stk.empty()){
if(root){
// res.push_back(cur->val); //Preorder
stk.push(root);
root = root->left;
}else{
root = stk.top(); stk.pop();
// res.push_back(root->val); //Inorder
root = root->right;
}
}
return res;
}
};
//Postorder
class Solution {
public:
vector<int> postorderTraversal(TreeNode* root) {
stack<TreeNode*> stk;
vector<int>res;
TreeNode* cur = root;
while(cur ||!stk.empty()){
if(cur){
res.push_back(cur->val);
stk.push(cur);
cur = cur->right;
}else{
cur = stk.top(); stk.pop();
cur = cur->left;
}
}
reverse(res.begin(), res.end());
return res;
}
};
//Morris: 流程图见, 094. Binary Tree Inorder Traversal.cpp
class Solution {
public:
vector<int> inorderTraversal(TreeNode* root) {
vector<int>res;
TreeNode* cur = root;
while(cur){
if(!cur->left)
{
//res.push_back(cur->val); //Inorder, preorder
cur = cur->right;
}else{
TreeNode* pre = cur->left;
while(pre->right && pre->right!=cur)
pre = pre->right;
if(pre->right){
//res.push_back(cur->val); //Inorder
pre->right = NULL;
cur = cur->right;
}
else{
//res.push_back(cur->val); //Pre-order
pre->right = cur;
cur = cur->left;
}
}
}
return res;
}
};
//PostOrder
class Solution {
public:
vector<int> postorderTraversal(TreeNode* root) {
vector<int>res;
TreeNode* cur = root;
while(cur){
if(!cur->right){
res.push_back(cur->val);
cur = cur->left;
}else{
TreeNode* pre = cur->right;
while(pre->left && pre->left != cur)
pre = pre->left;
if(pre->left){
pre->left = NULL;
cur = cur->left;
}else{
res.push_back(cur->val);
pre->left = cur;
cur = cur->right;
}
}
}
reverse(res.begin(), res.end());
return res;
}
};
//如果有right child, 经过root 两次
//112. Path Sum https://leetcode.com/problems/path-sum/
class Solution {
public:
bool hasPathSum(TreeNode* root, int sum) {
stack<TreeNode*>stk;
TreeNode* cur = root, *pre = nullptr;
while(cur || !stk.empty()){
if(cur){
stk.push(cur);
sum -= cur->val;
cur = cur->left;
}else{
cur = stk.top();
if(!cur->right && !cur->left && sum == 0)
return true;
if(cur->right && cur->right != pre){
cur = cur->right;
}
else{
pre = cur;
sum += cur->val;
cur = nullptr;
stk.pop();
}
}
}
return false;
}
};
//In order traversal, 不同于之前的iterative 解, 这是每个node 都先被push 进stack, 只有return back时候才pop
/*
详见 236. Lowest Common Ancestor of a Binary Tree
https://github.com/beckswu/Leetcode/blob/master/DFS/236.%20Lowest%20Common%20Ancestor%20of%20a%20Binary%20Tree.cpp#L32，
1
/
2
\
3
最上面的inorder 的 stack 到3时候是 [1，3
下面解的stack 是 到3是 [1,2,3]
*/
class Solution {
public:
TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
//iterative, path comparing
if(!root ) return root;
vector<TreeNode*> path;
if(root)
path.push_back(root); //temp是stack，元素是从root到现在node上的路径
TreeNode *prev = nullptr;
while(!path.empty()){
root=temp.back();
if(!root->left && !root->right || !root->right && prev==root->left || root->right && prev==root->right){
path.pop_back();
prev=root;
}
else{
if(!root->left || prev==root->left) path.push_back(root->right);
else path.push_back(root->left);
}
}
}
};
TreeNode* helper(TreeNode** head ){
int val = *head; //获取值, 比如值是5,
TreeNode** cur = head; //
head = &((*head)->left); //不会影响之前cur的值， 把head assign 一个新object, 之前绑定head的地址(cur)的object 并没有删除
*head = (*head)->left; //会影响cur的值， *head 取地址, 改变head这个地址绑定的值, 因为cur 和head地址一样 , 所以cur的值也改变
return cur;
}
//比如:
void helper(int *p){
int *newp = p;
*p = 5; //会影响newp的值, 因为newp 和 p地址一样，更改的p的值， newp自动会更改
p = new int(10); // 不会更改newp 的值, 因为p的地址被换掉了, 之前绑定p的地址并没有删除
}
| Title | Time | Space | Difficulty | Algorithm Note |
|---|---|---|---|---|
| 094. Binary Tree Inorder Traversal | O(n) | O(1) | Medium | 😍🔍Morris Traversal, 现在点连在 left-child 最右侧的node 右侧, 因为最右侧的node 最后visit |
| 099 Recover Binary Search Tree | O(n) | O(1) | Hard | 🔍😚 调换node 之间第一个最错误的（也是最大的prev），和最后一个错误（也是最小的cur），💡顺序一定是inorder，由小到大 |
| 144. Binary Tree Preorder Traversal | O(n) | O(1) | Medium | Morris Traversal,注意preorder 与inorder push 进vector的顺序的区别 |
| 145. Binary Tree Postorder Traversal | O(n) | O(1) | Hard | = 先right 再left 的 inorder traversal 🔍Morris Traversal |
| 208. Implement Trie (Prefix Tree) | O(n) | O(1) | Medium | Trie |
| 211. Add and Search Word - Data structure design | O(min(n, h)) | O(min(n, h)) | Medium | Trie + DFS |
| 226. Invert Binary Tree | O(n) | O(h), O(w)) | Easy | 👽 不可以 left = invert(right); right = invert(left);, 因为left 在invert right时候改变 |
| 297. Serialize and Deserialize Binary Tree | O(n) | O(h) | Hard | ✏️ostringstream & istringstream 用法, BFS－> pointer of pointer 存pointer 地址 |
| 307. Range Sum Query - Mutable | O(n), O(logn) | O(n) | Medium | ✏️ BIT & Segment Tree; BIT tree 需要arr作为参照物,每次根据val-arr[i]的update, update过后arr[i] = val |
| 525. Contiguous Array | O(n) | O(n) | Medium | 😍把所有的0变成-1， 所以当有sum[i,j] = 0时 => [i,j]中有同等的1 和 0， same as 325. Maximum Size Subarray Sum Equals k |
| 529. Minesweeper | O(m * n) | O(m + n) | Medium | ❌ 简单DFS |
| 538. Convert BST to Greater Tree | O(n) | O(h) | Easy | 😍注意Python BFS |
| 543. Diameter of Binary Tree | O(n) | O(h) | Easy | 🔍先尽可能dfs，再比较height 会更快 |
| 563. Binary Tree Tilt | O(n) | O(n) | Easy | ❌思路跟543. Diameter of Binary Tree 一样 |
| 572. Subtree of Another Tree | O(m * n) | O(h) | Easy | 😍 seralization |
| 606. Construct String from Binary Tree | O(n) | O(h) | Easy | ❌ Easy Recursion |
| 617. Merge Two Binary Trees | O(n) | O(h) | Easy | 😍 |
| 623. Add One Row to Tree | O(n) | O(h) | Medium | 😚 |
| 637. Average of Levels in Binary Tree | O(n) | O(h) | Easy | ❌ |
| 652. Find Duplicate Subtrees | O(n) | O(n) | Medium | 😍🔍 Seralization(String的memory 是 O(n^2)) / Hash, C++ 有定义hash. 注: 无须seralize 完整后再寻找, analysis |
| 653. Two Sum IV - Input is a BST | O(n) | O(h) | Easy | 😍🔍可以考怎么写 BST Iterator |
| 654. Maximum Binary Tree | O(n) | O(h) | Medium | 😍🔍💡 descending stack:
|
| 655. Print Binary Tree | O(n) | O(h) | Medium | Math找规律 |
| 662. Maximum Width of Binary Tree | O(n) | O(h) | Medium | ❌ Math 找规律, 逻辑跟655. Print Binary Tree类似 |
| 677. Map Sum Pairs | O(n) | O(t) | Medium | ❌Simple Trie |
| 684. Redundant Connection | O(n) | O(n) | Medium | 🔍Union Find 如果两个node 连接之前发现parent 已经一样，表示之前两个nodes已经连接，如果再连接edge，会构成cycle |
| 685. Redundant Connection II | O(n) | O(n) | Hard | Union Find 注意构成tree 的条件, 不能有一个child 连上两个parent, 然后去掉这个child一个链，保证都是一个child对应一个parent, 再看有没有cycle, 没有cycle表示去链去成功了, 有cycle 表示去链去错了 |
| 687. Longest Univalue Path | O(n) | O(h) | Easy | 😍 Really good Recussive Question! |
| 699. Falling Squares | O(nlogn) | O(n) | Hard | 😍Good Question! 若想找点属于 哪个范围之中 比如 3∈ (1,5) or (7,9) , 用map + binary search
|
| 814. Binary Tree Pruning | O(n) | O(h) | Medium | 😍Really good question! |
| 850. Rectangle Area II | O(nlogn) | O(h) | Hard | 🔍💡跟699. Falling Squares思路有点像, 根据height一层一层的算当层长方体面积,算完面积后更新上一层的底curx |
| 863. All Nodes Distance K in Binary Tree | O(n) | O(h) | Medium | 😍😍Really good question! 不必纠结于one pass, 需要child -> parent map |
| 865. Smallest Subtree with all the Deepest Nodes | O(n) | O(h) | Medium | 🔍 若left, right child 都是最深的, 则root为 最深的node |
| 889. Construct Binary Tree from Preorder and Postorder Traversal | O(n) | O(h) | Medium | 😍😍
|
| 1008. Construct Binary Search Tree from Preorder Traversal | O(n) | O(h) | Medium | 🎅Stack, Recursion, Morris Traversal |
| 1028. Recover a Tree From Preorder Traversal | O(n) | O(h) | Hard | 😚 stack / DFS, stack逻辑类似889. Construct Binary Tree from Preorder and Postorder Traversal |
| 1409. Queries on a Permutation With Key | O(nlogn) | O(n) | Medium | BIT, Fenwick Tree, 🎅 How to Build FenwickTree |
| Title | Time | Space | Difficulty | Algorithm Note |
|---|---|---|---|---|
| 007. Reverse Integer | O(1) | O(1) | Easy | |
| 009. Palindrome Number | O(1) | O(1) | Easy | |
| 012. Integer to Roman | O(n) | O(1) | Medium | |
| 013. Roman to Integer | O(n) | O(1) | Easy | |
| 964. Least Operators to Express Number | O(logn) | O(logn) | Hard | 🎅🎅🎅 |
| 1360. Number of Days Between Two Dates | O(1) | O(1) | Easy | |
| 1362. Closest Divisors | O(sqrt(n)) | O(1) | Medium | |
| 1363. Largest Multiple of Three | O(n) | O(1) | Hard | |
| 1390. Four Divisors | _O(n * sqrt(n)) _ | O(1) | Medium | ❌ |
| Title | Time | Space | Difficulty | Algorithm Note |
|---|---|---|---|---|
| 005.Longest Palindromic Substring | O(n) | O(n) | Medium | 🔍 manacher(马拉车算法), mx表示当前最长回文外右侧第一点, id是当前回文中心, p[i]表示当前最长回文, if i<mx, p[i] = min(p[2id-i], p[i]) |
| 006. ZigZag Conversion | O(n) | O(n) | Medium |
|
| 008. String to Integer (atoi) | O(n) | O(1) | Easy | C++可以用find_first_not_of |
| 014. Longest Common Prefix | O(n) | O(1) | Easy | loop所有数第0位到第i位，直到不相同,返回str[0].substr(0,i) |
| 028. Implement strStr() | O(n+k) | O(k) | Easy | kmp algorithm: prefix array[i]表示i点的最长的prefix 也是suffix长度 比如"ABA", 第三个a的最长的prefix 也是suffix 的长度是1 A 而prefix array[i], 作为index, 是当前最长prefix 也是suffix 的下一位 |
| 038. Count and Say | O(n * 2^n) | O(n2^n) | Easy | C++ find_if + bind1st |
| 043. Multiply Strings | O(m*n) | O(m+n) | Medium | C++ transform, 必须都从个位数(也就是string的最后一位开始算, 否则carry可能会超过10), back_inserter, 相当于按照原来从头到尾顺序push back |
| 058. Length of Last Word | O(n) | O(1) | Easy | C++ find if or find if + bind1st or string find_last_not_of + find_last_of |
| 067. Add Binary | O(n) | O(1) | Easy | string 加法, 跟415. Add Strings 和306. Addictive Number 类似 |
| 068. Text Justification | O(n) | O(1) | Hard | not a hard question, 跟725. Split Linked List in Parts 类似 |
| 125. Valid Palindrome | O(n) | O(1) | Easy | C++ 跳过非isalnum的 |
| 151. Reverse Words in a String | O(n) | O(1) | Medium | 先reverse所有的, 再reverse单个每个词, 记录总共len,最后用来截取, C++ find_first_not_of + find_first_of |
| 165. Compare Version Numbers | O(n) | O(1) | Medium | c++ 算当前version1,2的substr的数，如果其中一个碰到结尾，设当前数位0。 c, 可以用c_str() + strtol; python3 zip(*itertools.zip_longest(*splits, fillvalue=0)) |
| 214. Shortest Palindrome | O(n) | O(n) | Hard | 🔍可以把此题换一种问法: 以index0开始最长的部分palindrome 的长度, 部分最长的pal后面的掉个+s = 答案
|
| 242. Valid Anagram | O(n) | O(1) | Easy | 经典面试题 |
| 273. Integer to English Words | O(1) | O(1) | Hard | 无聊的recursion |
| 306. Addictive Number | O(n^3) | O(n) | Medium | recursion 从index0开始试所有的digit可能性直到成功, 比如开始是一位+两位, 还是三位+两位 , 需要一个string add的help function; python 可以用itertools.combination + startswith, 跟067. Add Binary 和415. Add Strings 类似, 只不过多个recursion |
| 383. Ransom Note | O(n) | O(n) | Easy | Hash map |
| 405. Convert a Number to Hexadecimal | O(n) | O(1) | Easy | 最后结果需要reverse，因为先插入最小的，注意负数的, -1>>4 = -1, 所以while加个条件 res.length()!=sizeof(int)*2 |
| 415. Add Strings | O(n) | O(1) | Easy | string加法，跟067. Add Binary 和306. Addictive Number 类似 |
| 420. Strong Password Checker | O(n) | O(1) | Hard | Brain Storm 详见C++ code 解释 |
| 434. Number of Segments in a String | O(n) | O(1) | Easy | 🔍, 根据s[i] 和 s[i-1]判断, or s[i] 和 s[i+1]判断 |
| 443. String Compression | O(n) | O(1) | Easy | two pointer + num reverse |
| 459. Repeated Substring Pattern | O(n) | O(n) | Easy | KMP |
| 468. Validate IP Address | O(1) | O(1) | Medium | 注意IPv4 和IPv6的定义(c++ code里), 判断一个char是不是符合十六进制用isxdigit(c) |
| 520. Detect Capital | O(1) | O(1) | Easy | C++ count_if; Python istitle()看是不是只有首字母大写 |
| 521. Longest Uncommon Subsequence I | O(min(a, b)) | O(1) | Easy | 题出的神经病，逗你玩儿 |
| 522. Longest Uncommon Subsequence II | _O(l * n^2) _ | O(1) | Medium | 🔍按照字母长度sort, 然后一个一个看str，有没有在list中有subsequence，没有的话, return 这个str长度,直到全部search完, return -1 or C++ equal_range + count_if , python 可以iter() |
| 524. Longest Word in Dictionary through Deleting | O((d * l) * logd) | O(1) | Medium | 按照字母长度sort,如果长度一样，按照alphabet sort, 找到第一个符合的 🔍python, max with key, min with key, filter, iter + next with default |
| 539. Minimum Time Difference | O(nlogn) | O(n) | Medium | C++ transform 把所有时间变分钟, 然后按minute sort, 答案就出自所有minute[i+1] - minute[i] or 1440 +minute[0] - minute.back() |
| 541. Reverse String II | O(n) | O(1) | Easy | |
| 551. Student Attendance Record I | O(n) | O(1) | Easy | |
| 556. Next Greater Element III | O(1) | O(1) | Medium | 可以用ascending stack or 两个for loop, 寻找i点往后最后一个比i点大的数(也是比i大,最接近i的数)(index j), swap(s[i], s[j]), 这样s[i]后面的数又大到小排序的, 把i往后的数到end全部reverse后变成Int, 就是答案, 跟031. Next Permutation思路类似 |
| 564. Find the Closest Palindrome | O(l) | O(l) | Hard | Brain Storm: 最接近的pal只可能5中选一, 100..001(l.size()+1), 99..99(l.size()-1), or string的前半部分 +1, +0, -1 加上前半部分的reverse(如果起始长度是奇数，reverse不包括前半部分最后一个，如果长度是偶数，都包括在内) |
| 591. Tag Validator | O(n) | O(n) | Hard | cdata 必须以 已 ]]>结束, recursion 找是不是valid tag, valid cdata, valid tagname |
| 647. Palindromic Substrings | O(n) | O(n) | Medium | 🔍 manacher(马拉车算法), 在snew中 p[i]表示以id为中心最长回文，到i点，res += p[i] /2 |
| 648. Replace Words | O(n) | O(t) | Medium | 🔍 Trie; python 可以用reduce + dict.__getitem__ |
| 657. Judge Route Circle | O(n) | O(1) | Easy | |
| 678. Valid Parenthesis String | O(n) | O(1) | Medium | 🔍Three Solutions
|
| 680. Valid Palindrome II | O(n) | O(1) | Easy | 🔍两个pointer, 检查s[i] == s[j]?, 遇到不等时，再看s[i+1, j], or s[i, j-1]是不是pal |
| 686. Repeated String Match | O(n+m) | O(n) | Easy | 🔍
|
| 696. Count Binary Substrings | O(n) | O(1) | Easy | manacher(马拉车)算法的变形 |
| 720. Longest Word in Dictionary | O(n) | O(t) | Easy | Trie or 先按长度sort, 长度越短, 排前面, loop word, loop s[i][0,len), 看是不是每个substr都在，都在话insert to hashset & update result |
| 722. Remove Comments | O(n) | O(k) | Medium | |
| 791. Custom Sort String | O(n) | O(k) | Medium | 可以当经典面试题, 三种解法:
|
| 796. Rotate String | O(n) | O(1) | Easy | 🔍两种kmp的解,
|
| 804. Unique Morse Code Words | O(n) | O(n) | Easy | Easy one unordered_set |
| 806.Number of Lines To Write String | O(n) | O(1) | Easy | Easy one but stupid question description |
| 809. Expressive Words | O(n+s) | O(1) | Medium | Two pointer: 如果word[i]!=S[j] 的时候， 看S的j-1, j, j+1是不是连续是三个，若不是，再看过去是不是连续三个，若不是，break |
| 816. Ambiguous Coordinates | O(n^3) | O(n) | Medium | 🔍选择小数点的关键是 不能左面位0， 右面结束也是0，比如00.1230不可以,但是即使左面等于0， 右面最后也不可以是0 |
| 819. Most Common Word | O(n+m) | O(m+n) | Easy | |
| 820. Short Encoding of Words | O(n) | O(t) | Medium |
|
| 824. Goat Latin | O(n + w^2) | O(l) | Easy | stringstream 作为string output |
| 831. Masking Personal Information | O(1) | O(1) | Easy | C++ transform 把所有字母都小写, s[0] 变成string 可以用 s.substr(0,1) or string(1,S[0]) |
| 833. Find And Replace in String | O(m+n) | O(n) | Medium | 先sort indexes,然后从后往前loop S,这样可以保持S前面的index不变, python 可以用zip + startswith |
| 839. Similar String Groups | O(n^2 * l) | O(n) | Easy | 🔍 Union Find Disjoint Set with Rank Heuristic, string 所在位置为index |
| 848. Shifting Letters | O(n) | O(1) | Medium | 加的时候及时%26, 小心overflow |
| 859. Buddy Strings | O(n) | O(1) | Easy | 判断条件: 1.长度不一样，false，2. 如果a == b，有没有重复的字母，有的话true, 没有false, 3, 如果不一样的位置个数不等于2, 或者a[diff[0]]!=b[diff[1]] or a[diff[1]]!=a[diff[1]] 返回false, 否则是true |
| 1374 Generate a String With Characters That Have Odd Count | O(n) | O(1) | Easy | ❌ |
| 1392. Longest Happy Prefix | O(n) | O(n) | Hard | KMP, Rolling Hash |
| 1408. String Matching in an Array | O(n) | O(n) | Easy | KMP, Rolling Hash |
| 1410. HTML Entity Parser | O(n) | O(t) | Medium | |
| 1417. Reformat The String | O(n) | O(1) | Easy | |
| summary |
|---|
reference reference2
|
| Title | Time | Space | Difficulty | Algorithm Note |
|---|---|---|---|---|
| 136. Single Number | O(n) | O(1) | Easy | 用xor ^, Python Reduce one line |
| 137. Single Number II | O(n) | O(1) | Medium | 🔍
|
| 190. Reverse Bits | O(1) | O(1) | Easy | 一位一位reverse bit, res每次向左移动一位，n向右移动一位 |
| 191. Number of 1 Bits | O(n) | O(1) | Easy | n = n & (n-1); |
| 201. Bitwise AND of Numbers Range | O(1) | O(1) | Medium | 一位一位比较digit，直到移动k位m=n, 那么此时的digit是bitwise and的结果, res = m<<k |
| 231. Power of Two | O(1) | O(1) | Easy | n = n & (n-1); |
| 260. Single Number III | O(n) | O(1) | Medium | 🔍两个pass,第一个pass, 通过Xor需要区分a 和 b的数 c(是a与b右面第一位不一样的数), 第二次pass, 通过c&nums[i]判断做xor, 找到a和b (binary 负数) |
| 268. Missing Number | O(n) | O(1) | Medium | Math, Swap, Xor |
| 318. Maximum Product of Word Lengths | O(n^2) | O(n) | Medium | 🔍可以用bit来判断两个string是不是有重合的字母, 用数字表示string, a是第一位被set，z是第26位被set, |
| 342. Power of Four | O(1) | O(1) | Easy | 4^n = (3+1)^n, 除了判断(n&n-1) , 还要判断n-1 是不是可以整除3 |
| 371. Sum of Two Integers | O(1) | O(1) | Easy | （a&b)<<1 表示需要相加进位的（两个1相加）, a ^ b 表示相加不进位（保留单个1） |
| 389. Find the Difference | O(1) | O(1) | Easy | 🔍找两个string唯一不同不同的char可以通过 xor |
| 393. UTF-8 Validation | O(n) | O(1) | Easy | 用count判断是不是起点, count==0 是起点 |
| 421. Maximum XOR of Two Numbers in an Array | O(nlogk) | O(n) | Medium | 🔍
|
| 461. Hamming Distance | O(1) | O(1) | Easy | 判断有多少bit, 与191. Number of 1 Bits和 231. Power of Two类似 |
| 462 Minimum Moves to Equal Array Elements II | O(nlogn) | O(1) | Medium | 🔍Meeting point, 先sort，然后逐个用最大减去最小, e.g [3,6,9], 不管中间点在哪，都要磨平9-3=6的差距 |
| 476. Number Complement | O(1) | O(1) | Easy | |
| 477. Total Hamming Distance | O(nlogk) | O(1) | Easy | 由第32位到第0位，loop每个bit，数当前bit位为1的个数为bitcount, 结果 res+= bitcount*(size-countsize), 与421. Maximum XOR of Two Numbers in an Array类似 |
| 645. Set Mismatch | O(n) | O(1) | Easy |
|
| 693. Binary Number with Alternating Bits | O(1) | O(1) | Easy | 🔍 |
| 762. Prime Number of Set Bits in Binary Representation | O(R-L) | O(1) | Easy | loop[L,R],数每个数多少个bit，因为log2(10^6) < 16, 事先把所有的prime存到hash set里面, 看现在bit数是不是质数，if so res++, 还可以用 __builtin_popcountl(n); bitset<32>(n).count() |
| C++ 0b表示binary number，比如0b10 = 2, 0b111 = 7 python 0b表示binary number，比如0b10 = 2, 0b111 = 7
|
| Title | Time | Space | Difficulty | Algorithm Note |
|---|---|---|---|---|
| 001 Two Sum | O(n) | O(n) | Easy | |
| 003. Longest Substring Without Repeating Characters | O(n) | O(n) | Medium | |
| 030. Substring with Concatenation of All Words | O((m+n)*k) | O(n*k) | Hard | 🔍k = word[0]长度, n = 整个words长度, m = S的长度。最快的解是两个map, map1记录words的每个string, |
| 036. Valid Sudoku | O(9*9) | O(9) | Medium | 🔍 用bit比较快，比如i在横着第2行出现, row[2] |
| 049. Group Anagrams | O(n * glogg) | O(n) | Medium | 经典 面试题, python list不能作为字典的key,但是tuple可以 |
| 076. Minimum Window Substring | O(n) | O(k) | Hard | 🔍sliding windows, 此题没有窗口的size，要去找最小的size，关键是如何确定window valid，记录每次滑到cur char也在T中出现的个数，当个数满足T.size(),证明window valid，然后逐步缩小start与i的距离，找最小点 |
| 149. Max Points on a Line | O(n^2) | O(n) | Hard | 每到一点，算跟别的点的斜率，注意1. 重合的点，2.斜率没有的定义的点, 在每一点都重新建一个hashmap |
| 187. Repeated DNA Sequences | O(n) | O(n) | Medium | 🔍 rolling hash (rabin-karp),
|
| 202. Happy Number | O(k) | O(k) | Easy | 要么是happy number，要么转化过程陷入循环 |
| 204. Count Primes | O(n) | O(n) | Easy | count从小往大后eliminate，注意要尽可能efficient |
| 205. Isomorphic Strings | O(n) | O(1) | Easy | 可以记录相同位置字母出现的上一位，或者把s,t字母相互关联起来, python 可以用find+map or zip+set |
| 217. Contains Duplicate | O(n) | O(n) | Easy | easy one, 可以用sort + unique |
| 219. Contains Duplicate II | O(n) | O(n) | Easy | easy one |
| 290. Word Pattern | O(n) | O(n) | Easy | 思路和205. Isomorphic Strings 一样 |
| 299. Bulls and Cows | O(n) | O(1) | Easy | One pass: 如果guess[i] 和 secret[i]一样, bull++, 不一样，++m[sec[i]], --m[guess[i]] python 可以用两个collectons.Counter相减, 得到重合的set |
| 336. Palindrome Pairs | O(n * k^2) | O(n*k) | Hard | 🔍trie |
| 387. First Unique Character in a String | O(n) | O(n) | Easy | 需要 two pass |
| 388. Longest Absolute File Path | O(n) | O(d) | Medium | map记录每一层现有的长度,到新的或者原来一层，更新map, res是max(map中含有“.”的一层), 用到string::find, string::find_first_not_of, std::find |
| 409. Longest Palindrome | O(n) | O(1) | Easy | 可以用std::count, 或者可以来回flip map, 当map位true +2 |
| 424. Longest Repeating Character Replacement | O(n) | O(1) | Medium | 🔍sliding window: 记录window的初始点, 如果当前长度 - 最大count > k, ++start(保持windows的最大长度), 如果满足，start不变，结果是s.size()-start |
| 438. Find All Anagrams in a String | O(n) | O(1) | Easy | sliding window: 跟567. Permutation in String思路一样
|
| 447. Number of Boomerangs | O(n^2) | O(n) | Easy | 可以用hypot |
| 454. 4Sum II | O(n^2) | O(n) | Medium | 可以把4sum看成two sum, 把A+B的和绑定，把C+D的和绑定，看-C-D是不是在A+B的map里 |
| 473. Matchsticks to Square | O(n * s * 2^n) | O(n * (2^n + s)) | Medium | DFS or Bit Mask |
| 523. Continuous Subarray Sum | O(n) | O(k) | Medium | 🔍求开始数到所有i的和的余数，如果现在这个的余数之前遇到过，表示，两个数之间有数的和正好整除k |
| 532. K-diff Pairs in an Array | O(n) | O(n) | Easy | 🔍one pass解: 两个hashset, lookup 和res, 找的时候既向上数又向下数, 为了避免重复, set(res)只push下限,结果就是res size |
| 554. Brick Wall | O(n) | O(m) | Meidum | 相当于求最多经过砖头缝缝 |
| 560. Subarray Sum Equals K | O(n) | O(k) | Medium | 🔍用hashmap记录每点的rolling sum(0,i)， 那么只需要找(0,i)的sum - k在不在map中，在的话 表示存在一点[0,j] + k = (0,i)的sum， res += map[sum-k] (可能一个sum出现很多遍) |
| 561. Array Partition I | O(n) | O(n) | Easy | Sort or Bucket Sort |
| 575. Distribute Candies | O(n) | O(n) | Easy | 对比set的长度和candies.size()/2的长度, C++可以用bitset |
| 594. Longest Harmonious Subsequence | O(n) | O(n) | Easy | |
| 599. Minimum Index Sum of Two Lists | O((m + n) * l) | O(m * l) | Easy | |
| 609. Find Duplicate File in System | O(n * k) | O(n * k) | Medium | |
| 721. Accounts Merge | O(nlogn) | O(n) | Medium | 🔍 Union Find, 不能用简单的hash table 找parent, 比如 (1@com, 2@com), (3@com, 4@com), (4@com, 2@com), 不用Union find分成两组 |
| 748. Shortest Completing Word | O(n) | O(1) | Medium | |
| 771. Jewels and Stones | O(n+m) | O(n) | Easy | |
| 811. Subdomain Visit Count | O(n) | O(n) | Easy | |
| 822. Card Flipping Game | O(n) | O(n) | Medium | 先把front[i]和end[i] 一样的插入到hash set, 再loop front & end, 选取不在hash set中最小的 |
| 825. Friends Of Appropriate Ages | O(n+k^2) | O(k) | Medium | 用hash map存age和count, loop两层hashmap, 判断内层和外层key是否满足条件, 满足的话更新结果 |
| 1418 Display Table of Food Orders in a Restaurant | O(n + tlogt + flogf) | O(n) | Medium | ✏️C++ transform |
| summary |
|---|
sliding windows: windows都是看以当前字母结尾的window. 比较对象s1, 被比较对象s2
|
// 438. Find All Anagrams in a String
//s2: "cbaebabacd" s1: "abc"
//Output: 0, 6]
//固定windows长度
class Solution {
public:
vector<int> findAnagrams(string s2, string s1) {
vector<int>map(26,0);
for(auto i: s1)
map[i-'a']++;
int len = s1.size(), count = s1.size();
vector<int>res;
for(int i = 0; i<s2.size();i++){
if(map[s2[i]-'a']-->0) count--;
if(count == 0) res.push_back(i-len+1);
if(i>=len-1){
if(++map[s2[i-len+1]-'a'] > 0) count++;
}
}
return res;
}
};
//不固定windows长度, 方法一:
class Solution {
public:
vector<int> findAnagrams(string s2, string s1) {
vector<int>map(26,0);
for(auto i: s1)
map[i-'a']++;
int len = s1.size(), left = 0;
vector<int>res;
for(int i = 0; i<s2.size();i++){
map[s2[i]-'a']--;
if(map[s2[i] - 'a'] <0)
while(left<= i && map[s2[i]-'a'] < 0) map[s2[left++]-'a']++;
/* or
//correct
while( mp[s2[left]]++ >= 0 )
++left;
++left;
//wrong: 比如 ab: eabc, left 会一直停在0(e)
while( mp[s2[left]] >= 0 )
mp[s2[left++]]++
*/
if(i-left+1 == len){
res.push_back(left);
}
}
return res;
}
};
//不固定windows长度 方法二:
class Solution {
public:
vector<int> findAnagrams(string s2, string s1) {
vector<int>map(26,0);
for(auto i: s1)
map[i-'a']++;
int len = s1.size(), left = 0;
vector<int>res;
for(int i = 0; i<s2.size();i++){
map[s2[i]-'a']--;
if(map[s2[i] - 'a'] <0)
while(map[s2[left++]-'a']++ >= 0);
//cout<<i <<" left "<<left<<endl;
if(i-left+1 == len){
res.push_back(left);
}
}
return res;
}
};
/*
"cbaebabacd"
"abc"
DEBUG stdout
0 left 0
1 left 0
2 left 0
3 left 4
4 left 4
5 left 4
6 left 5
7 left 6
8 left 6
9 left 10
*/
| Title | Time | Space | Difficulty | Algorithm Note |
|---|---|---|---|---|
| 567. Permutation in String | O(n) | O(1) | Medium | 😍sliding Window(长度为len(s1)), 每次移动框,vector减去新来的，加上刚刚pass的，直到l长度为0 |
| Title | Time | Space | Difficulty | Algorithm Note |
|---|---|---|---|---|
| 020. Valid Parentheses | O(n) | O(n) | Easy | ❌注意return true if stack is empty |
| 032. Longest Valid Parentheses | O(1) | O(n) | Hard | 😍
|
| 071. Simplify Path | O(n) | O(n) | Medium | ✏️ getline可以当做stack, 遇到".." stack pop; vector可以用作stack |
| 084. Largest Rectangle in Histogram | O(n) | O(n) | Hard | 😍
|
| 085. Maximal Rectangle | O(n*m) | O(m) | Hard | 🔍
|
| 101. Symmetric Tree | O(n) | O(h) | Easy | ❌ 注: iterative stack push 顺序 |
| 150. Evaluate Reverse Polish Notation | O(n) | O(n) | Medium | ✏️ Python Lambda Function in dictionary 🔍 C++ recursive solution |
| 155. Min Stack | O(n) | O(1) | Easy | 😚 Descending Stack: 两个stack,一个用来放正常的顺序，另一个作为min |
| 173. Binary Search Tree Iterator | O(1) | O(h) | Medium | 307. Range Sum Query - Mutable 逻辑类似, 不要先全部走完 |
| 232. Implement Queue using Stacks | O(1), amortized | O(n) | Easy | 🔍两个stack in & out, in用来push, top: 假如out为空，dump in to out |
| 224. Basic Calculator | O(n) | O(n) | Hard | ❌用sign=1记录+, -1记录减, 碰到num乘以res,'('res,sign push进stack, ')'先乘以stack的top(是sign),再加上stack的top(sign之前的res) |
| 227. Basic Calculator II | O(n) | O(n) | Medium | ❌ 用sign=1记录+, -1记录减, sign = 2 记录*， 3记录除, 上一个sign是乘或除，先进行operation |
| 331. Verify Preorder Serialization of a Binary Tree | O(n) | O(n) | Medium | 😍😍✏️stringstream + getline
|
| 341. Flatten Nested List Iterator | O(n) | O(h) | Medium | 😍😍stack + recursion从最后往前loop, queue从前往后loop, ✏️✏️C++/Python Iterator, 要存iterator, 不能存vector, 因为存vector memory会很大 |
| 385. Mini Parser | O(n) | O(h) | Medium | 遇到',' ']' 把之前的integer add， 比如[-1], [123,456], 遇到']',把现在这个nested list加入上个nested list |
| 394. Decode String | O(n) | O(h) | Medium | 🔍可以看看recursive 的解, 程序设计: 怎么设计一个好的stack, 类似726. Number of Atoms
|
| 456. 132 Pattern | O(n) | O(h) | Medium | 🎅🎅 寻找 s1 < s3 < s2，从后向前，Descending stack, 难点: 理解stack让s2 逐渐变大, 但s3可增也可减, 因为s2减小前就return true了 |
| 636. Exclusive Time of Functions | O(n) | O(n) | Medium | 🔍stack 存的是上个job的id |
| 682. Baseball Game | O(n) | O(n) | Easy | ❌bad problem description |
| 726. Number of Atoms | O(n^2) | O(n) | Hard | 类似 394. Decode String |
| 735. Asteroid Collision | O(n) | O(n) | Medium | 🎅 碰撞发生只能是新来的小于0，stack top > 0 |
| 736. Parse Lisp Expression | O(n) | O(n) | Hard | ❌stack需要两个，一个是存string dict(用来储存let的字典), 一个存string vector(用来储存上个string的split), 遇到'(', 如果之前是let, 先存map, 然后push进两个stack, string vector清空，字典不清空 。 遇到')', 算当前的, 把结果push到上个string(stkstring.top()) 的结尾, pop两个stack |
| 739. Daily Temperatures | O(n) | O(n) | Medium | 🔍 Ascending/Descending stack, 可以正向也可以反向 |
| 0901. Online Stock Span | O(n) | O(n) | Medium | |
| Ascending & Descending Stack 按照 container的顺序进行排序 |
| Title | Time | Space | Difficulty | Algorithm Note |
|---|---|---|---|---|
| 002. Add Two Numbers | O(n) | O(1) | Medium | |
| 021. Merge Two Sorted Lists | O(n) | O(1) | Easy | |
| 023. Merge k Sorted Lists | O(nklogk) | O(1) | Hard | Heap, Divide Conquer, 注: 不能用一直用0作为l 和r比，这样的话，l的size会增加的很快，到最后l size快成位nk了 |
| 024. Swap Nodes in Pairs | O(n) | O(1) | Easy | 建dummy, 提取往后第二个为nextnext，断第二，三链, nextnext后接上当前的next, 把nextnext接到当前的next, pt往后走两步 |
| 025. Reverse Nodes in k-Group | O(n) | O(1) | Hard | 类似206 Reverse Linked List |
| 061. Rotate List | O(n) | O(1) | Medium | |
| 082. Remove Duplicates from Sorted List II | O(n) | O(1) | Medium | |
| 083. Remove Duplicates from Sorted List | O(n) | O(1) | Easy | 不能用recusion, recursion的话会用pass n回linked list，用O(n)space, iterative解space只用O(1),tree可以用recursion原因是它的stack space是O(logn) |
| 138. Copy List with Random Pointer | O(n) | O(1) | Medium | 1. 先把每个node复制一个，把复制的贴在被复制的后面 2. loop node(现在长度是2n), 把cur->next->random = cur->random->next，因为cur->random->next是复制cur->random过来的 3. 最后结果就是把每个偶数位的node连接起来，同时要消除偶数的node(长度由2n变回n)，否则结果是修改了原来的node |
| 147. Insertion Sort List | O(n^2) | O(1) | Medium | Sort |
| 148. Sort List | O(nlogn) | O(logn) | Medium | Sort
|
| 160. Intersection of Two Linked Lists | O(n+m) | O(1) | Easy | 利用的是 lA + lB_1 = lA_1 + lB (lenA + B交点前的长度 = lenB + A交点前的长度), pA，pB 每次都前进一步，pA到end,pA设为Bhead, pB到end,pB设为Aend, 这种尾对头只换一次，第二次pA 或者pB到end 返回NULL(就是没有交点) |
| 203. Remove Linked List Elements | O(n) | O(1) | Easy | |
| 206. Reverse Linked List | O(n) | O(1) | Easy | |
| 234. Palindrome Linked List | O(n) | O(1) | Easy | revese list前面部分，然后reverse之后，逐个对比前半部分是否等于后半部分的值 |
| 237. Delete Node in a Linked List | O(n) | O(1) | Easy | 把node->next的val提到node val然后delete node->next |
| 328. Odd Even Linked List | O(n) | O(1) | Medium |
|
| 445. Add Two Numbers II | O(n+m) | O(m+n) | Medium | 用两个stack,把每个list值push 进stack，最后push进的先算 |
| 725. Split Linked List in Parts | O(n) | O(1) | Medium | 每次前进到此次push进vector的最后一位, 然后断链, 第i个vector长度为 n//k + (i< n%k) |
| 817. Linked List Components | O(n+m) | O(m) | Medium | 把vector转化成unordered_set, 然后判断每个val,是不是在unordered_set里面 |
| LinkedList 当head, cur 指向同一点, cur = cur->next; | head 不会改变, 但是当cur在head之后，head包含cur, cur = cur->next, head会跳过cur这点 | |||
| two pointer 1.whiLe(fast->next && fast->Next->next) 是找中点, 比如1-2-3-4-5-6，slow最后等于3 2.whiLe(fast && fast->Next) 是找中后一点,比如1-2-3-4-5-6，slow最后等于4, 1-2-3-4-5 最后是3 |
| Title | Time | Space | Difficulty | Algorithm Note |
|---|---|---|---|---|
| 239. Sliding Window Maximum | O(n) | O(k) | Hard | 😍 Monoqueue using Deque
|
| Title | Time | Space | Difficulty | Algorithm Note |
|---|---|---|---|---|
|
||||
| 264. Ugly Number II | O(n) | O(1) | Medium | 😍🎅🎅
|
| 295. Find Median from Data Stream | O(nlogn) | O(1) | Medium | 虽是hard, 逻辑简单, 两个heap, minheap, maxheap, ✏️可以看看python heapq用法 heappushpop |
| 313. Super Ugly Number | O(n*k) | O(n+k) | Medium | 类似 264. Ugly Number II |
| 373. Find K Pairs with Smallest Sums | O(k * log(min(n, m, k))) | O(min(n, m, k)) | Medium | 🔍注: 不用hashset, 可不重复push 进heap 🎅不能用two pointer, 比如 [1,7], [2,6], 结果是[1,2],[1,6],[2,7], two pointer给的是[1,2],[1,6],[6,7] ✏️: python itertools.product, itertools.islice O(k) solution |
| 378. Kth Smallest Element in a Sorted Matrix | O(k * log(min(n, m, k))) | O(min(n, m, k)) | Medium | Binary Search, Heap, ZigZag Search |
| 407. Trapping Rain Water II | O(m * n * (logm + logn)) | O(m*n) | Hard | 😍🎅
|
| 632. Smallest Range Covering Elements from K Lists | O(nklogk) | O(k) | Hard | 😍🎅
|
| 846. Hand of Straights | O(nlogn) | O(n) | Medium | 🔍
|
| 846. Hand of Straights | O(nlogn) | O(n) | Hard | |
| 973. K Closest Points to Origin | O(n) average | O(1) | Easy | ✏️Quick-Select |
| 1046. Last Stone Weight | O(nlogn) | O(n) | Easy | |
Two pointer 用于
- detect cycle
- sorted array比大小,一个array一个pointer
- linked list找到middle point
| Title | Time | Space | Difficulty | Algorithm Note |
|---|---|---|---|---|
| 019. Remove Nth Node From End of List | O(n) | O(1) | Medium | 🔍two pointer, list总长l, 需要remove的index是l-n, slow要前进到l-n-1, 所以先前进n个，再前进到尾部就是l-n-1 |
| 086. Partition List | O(n) | O(1) | Medium | 🔍
|
| 141. Linked List Cycle | O(n) | O(1) | Easy | ❌ |
| 142. Linked List Cycle II | O(n) | O(1) | Medium | 🔍具体数学解释, 类似287. Find the Duplicate Number |
| 143. Reorder List | O(n) | O(1) | Medium | 😚🎅 用fast & slow先找到medium的点，slow到结尾所有的点reverse, 然后p1 = head, p2 = middle后一点，一个p1, 一个p2 插进pointer，就是结果 |
| 167.Two Sum II - Input array is sorted | O(n) | O(1) | Easy | ❌ two pointer, 一个从开始位置，一个从末尾的位置 |
| 283. Move Zeroes | O(n) | O(1) | Easy | ❌ 记录swap后第一个0位置 |
| 287. Find the Duplicate Number | O(n) | O(1) | Easy | 😍🎅 类似142. Linked List Cycle II ,有duplicate一定会有cycle, 难点: 找起点
|
| 344. Reverse String | O(n) | O(1) | Easy | 🔍bit来进行swap |
| 349. Intersection of Two Arrays | O(n+m) | O(min(m, n)) | Easy |
|
| 350.Intersection of Two Arrays II | O(n+m) | O(1) | Easy | ❌
|
| 457. Circular Array Loop | O(n) | O(1) | Medium | ❌array loop必须是单向的, 比如1->2, 2->1 不算是loop，循环array每次两个pointer检查有没有loop,如果没有loop,把这次所有走过的点都标成0,下次不用再走了, 类似141. Linked List Cycle |
| 611. Valid Triangle Number | O(n^2) | O(1) | Medium | 🎅(无法达到O(n))，先sort然后两个pointer,每一个都指向一个边, |
| 777. Swap Adjacent in LR String | O(n) | O(1) | Medium | 🎅 难点是: 寻找left 和 right. R是向前走，L是向后走（swap R 和L都需要X）, 两个pointer，遇到X往前跳 |
| 826. Most Profit Assigning Work | O(mlogm + nlogn) | O(1) | Medium | 😍🔍
|
| 828. Unique Letter String | O(n) | O(1) | Hard | 😍🎅
|
| 838. Push Dominoes | O(n) | O(n) | Medium | 🎅🎅🎅 |
| 844. Backspace String Compare | O(m+n) | O(1) | Easy | 两个pt，都从s,t 从后往前对比 |
| 986. Interval List Intersections | O(m+n) | O(1) | Medium |
| Title | Time | Space | Difficulty | Algorithm Note |
|---|---|---|---|---|
| 056. Merge Intervals | O(nlogn) | O(n) | Medium | 类似的题 |
| 057. Insert Interval | O(nlogn) | O(n) | Hard | 类似的题 |
| 075. Sort Colors | O(n) | O(1) | Medium | 🎅 Tri Partition, 小的放前面, 大的放最后 |
| 088. Merge Sorted Array | O(n) | O(1) | Easy | ❌从后往前 |
| 147. Insertion Sort List | O(n^2) | O(1) | Medium | 跟148. Sort List 一样 |
| 148. Sort List | O(nlogn) | O(logn) | Medium |
|
| 164. Maximum Gap | O(n) | O(n) | Hard | 😍🔍
|
| 164. Maximum Gap | O(n) | O(n) | Hard | 😍🔍
|
| 179. Largest Number | O(nlogn) | O(n) | Medium | ✏️✏️ Python Lambda Sort |
| 218. The Skyline Problem | O(nlogn) | O(logn) | Hard | 😍😍 priority_queue or multiset(看critical point) |
| 274. H-Index | O(n) | O(n) | Medium | ❌counting Sort |
| 315. Count of Smaller Numbers After Self | O(nlogn) | O(n) | Hard | MergeSort, BIT |
| 324. Wiggle Sort II | O(n) average | O(1) | Medium | ❌(1 + 2*index) % (n | 1)保证median左面数map奇数位，mediam右面的数map偶数位
|
| 327. Count of Range Sum | O(nlogn) | O(n) | Hard | MergeSort with Count, BIT |
| 347. Top K Frequent Elements | O(n) | O(n) | Medium | 😍 Bucket Sort, Quick Select,
|
| 406. Queue Reconstruction by Height | O(n * sqrt(n))~O(n^2) | O(n) | Medium | 😚 关键是认清sort的顺序 先把height大的安排了，如果height一样再sort k有小到大。 sqrt(n)解是一样的sort，但是把sort之后的插入到不同的组中，每个组不超过sqrt(n)个元素 |
| 462. Minimum Moves to Equal Array Elements II | O(nlogn) | O(n) | Medium | Medium是最小化Sum of Absolute Deviations; Quick Select: O(n) on average |
| 451. Sort Characters By Frequency | O(n) | O(n) | Medium | Bucket Sort, Quick Select(n-th elements) O(nlogn), priority_queue O(nlogn) |
| 692. Top K Frequent Words | O(nlogk) | O(n) | Medium | Bucket Sort, Quick Select(n-th elements), priority_queue |
| 853. Car Fleet | O(nlogn) | O(n) | Medium | Greedy: sort postion又大到小，再sort到target的时间由小到大 |
| 1029. Two City Scheduling | O(n) average | O(n) | Easy | Greedy**,quick select |
| C++priority_queue<pair<int,int>>pq 先对比first, top是first最大的， constructor: greater<int>是让top返回最小的数,大的数放后面 |
python的heappop()先pop对比first,then second, top是first最小的 | |||
| 1365 How Many Numbers Are Smaller Than the Current Number | O(n+m) | O(m) | Easy | 🔍O(n+m) Solution Counting Sort 🔍Python Counting Sort |
| 1366. Rank Teams by Votes | O(n) | O(1) | Medium | 🔍Python Sort list based on Dictonary value |
| Title | Time | Space | Difficulty | Algorithm Note |
|---|---|---|---|---|
| 095. Unique Binary Search Trees II | O(4^n / n^(3/2)) | O(4^n / n^(3/2)) | Medium | 😍 🔍loop start -> end. Generate vector of left subtree 和right subtree, 然后排列组合把他们弄在一起 |
| 096. Unique Binary Search Trees | O(n) | O(1) | Medium | DP, cartesian product 作为root，sum(#left + #right) Catalan number |
| 098. Validate Binary Search Tree | O(n) | O(1) | Medium | 用prev 点, iterative + recurssion |
| 100. Same Tree | O(n) | O(h) | Easy | ❌ |
| 104. Maximum Depth of Binary Tree | O(n) | O(h) | Easy | ❌ |
| 105. Construct Binary Tree from Preorder and Inorder Traversal | O(n) | O(h) | Medium | 😍😍 🔍🔍注意和889. Construct Binary Tree from Preorder and Postorder Traversal stack的区别. preorder 第一个是tree的root, inorder 中root->val左面root的left tree,右面root的right tree, |
| 106. Construct Binary Tree from Inorder and Postorder Traversal | O(n) | O(h) | Medium | 😍😍 🔍🔍 O(1) memory 和 stack |
| 108. Convert Sorted Array to Binary Search Tree | O(n) | O(logn) | Easy | ❌ 跟095. Unique Binary Search Trees II逻辑一样 binary tree height 需要balanced |
| 109. Convert Sorted List to Binary Search Tree | O(n) | O(logn) | Medium | 🔍注意O(N)的解，不是two pointer的 |
| 110. Balanced Binary Tree | O(n) | O(h) | Medium | 😚 跟095. Unique Binary Search Trees II类似 用bottom-up 比top-down 更efficient |
| 111. Minimum Depth of Binary Tree | O(n) | O(h) | Medium | ❌if not left: return h(r.right)+1; , if not right: return h(r.left)+1; else: return min(h(r.right), h(r.left))+1; |
| 114. Flatten Binary Tree to Linked List | O(n) | O(h) | Medium | 🔍😍 preorder 的reverse |
| 116. Populating Next Right Pointers in Each Node | O(n) | O(1) | Medium | 审题: perfect binary tree
|
| 124. Binary Tree Maximum Path Sum | O(n) | O(h) | Hard | 🔍not hard question |
| 129. Sum Root to Leaf Numbers | O(n) | O(h) | Medium | O(1) extra memory |
| 241. Different Ways to Add Parentheses | O(n* 4^n / n^(3/2)) | O(n * 4^n / n^(3/2)) | Medium | 😍 算sign前的，算sign后的,然后做前和后的permutation和 |
| 337. House Robber III | O(n) | O(h) | Medium | 🎅Greedy Algorithm. 返回vector, vector[0]存的是用上一个最大的获取值，vector[1]存的是不用上一个 最大的获取值 |
| 395. Longest Substring with At Least K Repeating Characters | O(n) | O(n) | Medium | 😍😍
|
| 404. Sum of Left Leaves | O(n) | O(h) | Easy | ❌ |
| 437. Path Sum III | O(n) | O(h) | Easy | 🔍一定用unorderedmap , 不能用unordered_set, 比如 -5,5,-6,6,4, 要sum = 4, 可以从-5到4 或者-6 到4 |
| 669. Trim a Binary Search Tree | O(n) | O(h) | Easy | 😚 |
| 671. Second Minimum Node In a Binary Tree | O(n) | O(h) | Easy | ❌ |
| 761. Special Binary String | O(n^2) | O(n) | Hard | ❌Bad problem description Divide-conquer, 把每个special string 再分成小的special string,然后sort |
| Title | Time | Space | Difficulty | Algorithm Note |
|---|---|---|---|---|
| 004. Median of Two Sorted Arrays | O(log(min(m, n))) | O(1) | Hard | 💜🎅🎅 |
| 033. Search in Rotated Sorted Array | O(log(n)) | O(1) | Medium | 💜🎅Similar Question: |
| 034. Find First and Last Position of Element in Sorted Array | O(log(n)) | O(1) | Medium | lowerbound/upperbound/EqualRange, lowerbound 可以convert to int |
| 35. Search Insert Position | O(log(n)) | O(1) | Easy | |
| 069. Sqrt(x) | O(log(n)) | O(1) | Easy | 🎅 Bit Solution similar Question |
| 074. search a 2D Matrix | O(logm + logn) | O(1) | Medium | lower_bound, upper_bound lambda |
| 081. Search in Rotated Sorted Array II | O(logn) | O(1) | Medium | 💜🎅 Similar Question: |
| 153. Find Minimum in Rotated Sorted Array | O(logn) | O(1) | Medium | Similar Question: |
| 154. Find Minimum in Rotated Sorted Array II | O(logn) ~ O(n) | O(1) | Hard | Similar Question: |
| 162. Find Peak Element | O(logn) | O(1) | Medium | ❌ |
| 222. Count Complete Tree Nodes | O((logn)^2) | O(1) | Medium | 注意审题 complete tree |
| 275. H-Index II | O(logn) | O(1) | Medium | |
| 278. First Bad Version | O(logn) | O(1) | Easy | |
| 300. Longest Increasing Subsequence | O(nlogn) | O(n) | Medium | 💜🎅🎅🎅 similar question |
| 354. Russian Doll Envelopes | O(nlogn) | O(n) | Hard | 💜🎅similar question |
| 363. Max Sum of Rectangle No Larger Than K | O(min(m, n)^2 * max(m, n) * logn(max(m, n))) | O(max(m, n)) | Hard | 💜🎅🎅, 利用Set |
| 367. Valid Perfect Square | O(logn) | O(1) | Easy | Similar Question |
| 374. Guess Number Higher or Lower | O(logn) | O(1) | Easy | |
| 378. Kth Smallest Element in a Sorted Matrix | O(n * log(MAX - MIN) | O(1) | Medium | l=m[0][0], r=m[-1][-1], binary search 是否至少有k个数小于等于mid |
| 410. Split Array Largest Sum | O(nlogn) | O(1) | Hard | 💜 |
| 436. Find Right Interval | O(nlogn) | O(n) | Medium | map lower bound |
| 475. Heaters | O((m + n) * logn) | O(1) | Easy |
|
| 540. Single Element in a Sorted Array | O(logn) | O(1) | Medium | |
| 658. Find K Closest Elements | O(logn+k) | O(1) | Medium | x-arr[left-1]<=arr[right]-x 保证left一定是起点，right是最后数后面一位 |
| 668. Kth Smallest Number in Multiplication Table | O(log(mn)*min(n,n)) | O(1) | Hard | binary search [1,m*n], isValid判断是否有至少有k个element在乘法表中 |
| 719. Find K-th Smallest Pair Distance | O(nlogn + nlogw) | O(1) | Hard | sort nums, l=0, r = nums[-1]-nums[0], binary search是否有k个数大于等于mid在num中 |
| 744. Find Smallest Letter Greater Than Target | O(logn) | O(1) | Easy | 判断最后一个字母是不是大于target, 大于的话用upperbound，否则返回第一个char |
| 786. K-th Smallest Prime Fraction | O(nlogr) | O(1) | Hard |
|
| 793.Preimage Size of Factorial Zeroes Function | O((logk)^2) | O(1) | Hard | l = 0, r=5*k, binary search mid是否有k个零的0，有的话r=mid, 否则l = mid+1, 最后再判断l是否有k个0, 有的话返回5,没有的话返回0 |
| 1385. Find the Distance Value Between Two Arrays | O((n + m) * logm) | O(1) | Easy | 🎅Binary Search, Two pointer |
| Title | Time | Space | Difficulty | Algorithm Note |
|---|---|---|---|---|
| 220. Contains Duplicate III | O(nlogn) | O(n) | Medium | set/multiset lower_bound 或者python OrderedDict, 每次popitem(false) pop 最先insert的 |
| 230 Kth Smallest Element in a BST | O(max(h, k)) | O(min(h, k)) | Medium | inorder traversals(从最小的travel到最大的) / stack |
| 235. Lowest Common Ancestor of a Binary Search Tree | O(h) | O(1) | Easy | 利用 binary search tree的性质 |
| 352. Data Stream as Disjoint Intervals | O(logn) | O(n) | Hard | |
| 449. Serialize and Deserialize BST | O(n) | O(h) | Medium | preorder traversals |
| 450. Delete Node in a BST | O(h) | O(h) | Medium |
|
| 530. Minimum Absolute Difference in BST | O(n) | O(h) | Easy | 利用binary search tree的性质 或者inorder traversal 带着前面prev的node val |
| 783. Minimum Distance Between BST Nodes | O(n) | O(h) | Easy | 利用binary search tree的性质 或者inorder traversal 带着前面prev的node val(与530题 解法一样) |
| 1382 Balance a Binary Search Tree | O(n) | O(h) | Medium |
| Title | Time | Space | Difficulty | Algorithm Note |
|---|---|---|---|---|
| 094. Binary Tree Inorder Traversal | O(n) | O(1) | Medium | Tree |
| 095. Unique Binary Search Trees II | O(4^n / n^(3/2)) | O(4^n / n^(3/2)) | Medium | 😍 🔍loop start -> end. Generate vector of left subtree 和right subtree, 然后排列组合把他们弄在一起 |
| 098. Validate Binary Search Tree | O(n) | O(1) | Medium | 用prev 点, iterative + recurssion |
| 099 Recover Binary Search Tree | O(n) | O(1) | Hard | Tree |
| 100. Same Tree | O(n) | O(h) | Easy | ❌ |
| 104. Maximum Depth of Binary Tree | O(n) | O(h) | Easy | ❌ |
| 101. Symmetric Tree | O(n) | O(h) | Easy | ❌ 注: iterative stack push 顺序 |
| 108. Convert Sorted Array to Binary Search Tree | O(n) | O(logn) | Easy | ❌ 跟095. Unique Binary Search Trees II逻辑一样 binary tree height 需要balanced |
| 109. Convert Sorted List to Binary Search Tree | O(n) | O(logn) | Medium | 🔍注意O(N)的解，不是two pointer的 |
| 110. Balanced Binary Tree | O(n) | O(h) | Medium | 😚 跟095. Unique Binary Search Trees II类似 用bottom-up 比top-down 更efficient |
| 111. Minimum Depth of Binary Tree | O(n) | O(h) | Medium | ❌if not left: return h(r.right)+1; , if not right: return h(r.left)+1; else: return min(h(r.right), h(r.left))+1; |
| 112. Path Sum | O(n) | O(h) | Easy | 🔍(iterative Solution: 如果有right会经过root 两次)[https://github.com/beckswu/Leetcode/blob/master/DFS/112.%20Path%20Sum.cpp#L74] |
| 124. Binary Tree Maximum Path Sum | O(n) | O(h) | Hard | 🔍not hard question |
| 129. Sum Root to Leaf Numbers | O(n) | O(h) | Medium | O(1) extra memory |
| 144. Binary Tree Preorder Traversal | O(n) | O(1) | Medium | Tree |
| 145. Binary Tree Postorder Traversal | O(n) | O(1) | Hard | Tree |
| 199 Binary Tree Right Side View | O(n) | O(h) | Medium | ❌ Easy |
| 222. Count Complete Tree Nodes | O((logn)^2) | O(1) | Medium | |
| 257 Binary Tree Paths | O(n * h) | O(h) | Easy | ❌ Easy |
| 404. Sum of Left Leaves | O(n) | O(h) | Easy | ❌ |
| 437. Path Sum III | O(n) | O(h) | Easy | unorderedmap 存的在现在点之前的 <prefix sum, frequency> pairs. 从中间某点到现在sum = 从root到现在点sum - root到中间某点的sum |
| 515. Find Largest Value in Each Tree Row | O(n) | O(h) | Medium | ❌ DFS / BFS |
| 538. Convert BST to Greater Tree | O(n) | O(h) | Easy | Tree |
| 543. Diameter of Binary Tree | O(n) | O(h) | Easy | Tree |
| 572. Subtree of Another Tree | O(m * n) | O(h) | Easy | |
| 617. Merge Two Binary Trees | O(n) | O(h) | Easy | |
| 623. Add One Row to Tree | O(n) | O(h) | Medium | |
| 637. Average of Levels in Binary Tree | O(n) | O(h) | Easy | |
| 653. Two Sum IV - Input is a BST | O(n) | O(h) | Easy | |
| 671. Second Minimum Node In a Binary Tree | O(n) | O(h) | Easy | ❌ |
| 687. Longest Univalue Path | O(n) | O(h) | Easy | |
| 669. Trim a Binary Search Tree | O(n) | O(h) | Easy | 😚 |
| 814. Binary Tree Pruning | O(n) | O(h) | Medium | |
| 863. All Nodes Distance K in Binary Tree | O(n) | O(h) | Medium | 😍😍Really good question! 不必纠结于one pass, 需要child -> parent map |
| 865. Smallest Subtree with all the Deepest Nodes | O(n) | O(h) | Medium | 🔍DFS, left level == right level 返回root, if left level > right level, 返回left dfs的node else返回right dfs的 |
| 889. Construct Binary Tree from Preorder and Postorder Traversal | O(n) | O(h) | Medium | 😍😍
|
| 1008. Construct Binary Search Tree from Preorder Traversal | O(n) | O(h) | Medium | 🔍Stack / Morris Traversal |
| 1028. Recover a Tree From Preorder Traversal | O(n) | O(h) | Hard | 😚 stack / DFS, stack逻辑类似889. Construct Binary Tree from Preorder and Postorder Traversal |
| 1367 Linked List in Binary Tree | O(n+l) | O(h+l) | Medium | |
| Title | Time | Space | Difficulty | Algorithm Note |
|---|---|---|---|---|
| 112. Path Sum | O(n) | O(h) | Easy | 🔍iterative Solution: 如果有right会经过root 两次 |
| 113 Path Sum II | O(n) | O(h) | Medium | 🔍iterative Solution: 如果有right会经过root 两次 |
| 199 Binary Tree Right Side View | O(n) | O(h) | Medium | ❌ Easy |
| 200 Number of Islands | O(m * n) | O(m * n) | Medium | ✏️ Union Find with Rank Heuristics, ✏️Python Complex number 表示四个DFS 方向 |
| 236 Lowest Common Ancestor of a Binary Tree | O(n) | O(h) | Medium | 🔍 Iterative Solution |
| 257 Binary Tree Paths | O(n * h) | O(h) | Easy | ❌ Easy |
| 282 Expression Add Operators | O(4^n) | O(n) | Hard | 🎅难点: 弄清 cv (cumulative sum), pv(previous sum) 关系, pos 到现在process的index，注意:
|
| 301. Remove Invalid Parentheses | O(C(n, c)) | O(c) | Hard | 😍😍Complexity Analysis
|
| 329. Longest Increasing Path in a Matrix | O(m * n) | O(m * n) | Hard | 😍
|
| 332. Reconstruct Itinerary | O(t! / (n1! * n2! * ... nk!)) | O(t) | Medium | 😍
|
| 399. Evaluate Division | O(q*|V|!) | O(e) | Medium | DFS with memiozation 用unordered_map, vector, unordered_set记录是否经过， 跟329. Longest Increasing Path in a Matrix类似 |
| 417. Pacific Atlantic Water Flow | O(m * n) | O(m * n) | Medium | 😍🎅 bit mask, 难点: 起点必须是四个边 |
| 440. K-th Smallest in Lexicographical Order | O(logn) | O(logn) | Hard | 找规律, 不能一个一个算, 要跳缩减区间 |
| 464. Can I Win | O(n!) | O(n) | Medium | 😚DFS+Memoization 难点: Memoization记录的不能是还剩多少到target, 记录是现在可选择的set能不能赢 |
| 515. Find Largest Value in Each Tree Row | O(n) | O(h) | Medium | ❌ DFS / BFS |
| 547. Friend Circles | O(n^2) | O(n) | Medium | Union Find with Rank Heuristic / DFS |
| 638. Shopping Offers | O(n * 2^n) | O(n) | Medium | 🎅✏️ [设计一个好的DFS structure](https://github.com/beckswu/Leetcode/blob/master/DFS/638.%20Shopping%20Offers.cpp#L42] |
| 690. Employee Importance | O(n) | O(h) | Easy | 需要用unordered_map, 因为vector index 不等同于 id |
| 695. Max Area of Island | O(m*n) | O(m*n) | Medium | ✏️Python Complex number 表示四个DFS 方向 |
| 733. Flood Fill | O(m*n) | O(m*n) | Easy | ❌ |
| 749. Contain Virus | O((m * n)^(4/3)) | O(m * n) | Hard | 😚 DFS/BFS, every step try each possibility see where is max new Infection area, then build wall and update grid |
| 753. Cracking the Safe | O(k^n) | O(k^n) | Hard | 🎅 Greedy + BrainStorming, 难点:如果设置起始数字，如何Loop 不会有deadlock |
| 756. Pyramid Transition Matrix | O(a^b) | O(a^b) | Medium | bottom-up, bit mask |
| 785. Is Graph Bipartite? | O(|V+E|) | O(|V|) | Medium | DFS/BFS + Bit Mask, 用红蓝两色表vertex，如果相邻的node 一样颜色 return false |
| 797. All Paths From Source to Target | O(p + r * n) | O(n) | Medium | ❌ |
| 802. Find Eventual Safe States | O(|V+E|) | O(|V|) | Medium | 😚 DFS + bit mask 需要定义state 0：unvisited, 1 visited not safe, 2 visited not safe, 3 visited and safe 注意不能用visited 的value 代替boolean 的value |
| 886. Possible Bipartition | O(|V+E|) | O(|V+E|) | Medium | DFS, BFS |
| 980. Unique Paths III | O((m * n) * 2^(m * n)) | O((m * n) * 2^(m * n)) | Medium | DFS, BFS |
| 1367 Linked List in Binary Tree | O(n+l) | O(h+l) | Medium | KMP 🔍C++ 用const auto [] get function return pair |
| 1368 Minimum Cost to Make at Least One Valid Path in a Grid | O(m*n) | O(m*n) | Medium | BFS + DFS |
| 1377. Frog Position After T Seconds | O(n) | O(n) | Hard | ✏️Python Set |
| 1391. Check if There is a Valid Path in a Grid | O(m*n) | O(1) | Medium | |
| 1397. Count Number of Teams | O(m*n) | O(m) | Hard | DFS /DP + KMP 🎅🎅 |
| Title | Time | Space | Difficulty | Algorithm Note |
|---|---|---|---|---|
| 017. Letter Combinations of a Phone Number | O(n * 4^n) | O(n) | Medium | ✏️Python Lambda Function |
| 022. Generate Parentheses | O(4^n / n^(3/2)) | O(n) | Medium | ✏️Python Trick |
| 037. Sudoku Solver | O((9!)^9) | O(1) | Hard | 可以用bit mask |
| 039. Combination Sum | O(k * n^k) | O(k) | Medium | ✏️Python Trick, Difference between yield and return |
| 040. Combination Sum II | O(n * n!) | O(n) | Medium |
|
| 216. Combination Sum III | O(k * C(n, k)) | O(k) | Medium | 🔍Python itertools.combination |
| 046. Permutations | O(\n * n!) | O(n) | Medium | |
| 047. Permutations II | O(\n * n!) | O(n) | Medium | |
| 051. N-Queens | O(n!) | O(n) | Hard | 🔍Python Solution |
| 052. N-Queens-II | O(n!) | O(n) | Hard | ❌ 与051. N-Queens 逻辑一样 |
| 077. Combinations | O(k * C(n, k)) | O(k) | Medium | 😍 Python Multiple Solution |
| 079. Word Search | O(m * n * l) | O(l) | Medium | Simple DFS. smart way: 把visited的字母改了 省掉了hashset, visited |
| 093. Restore IP Addresses | O(1) | O(1) | Medium | recursive & iterative |
| 078. Subsets | O(n * 2^n) | O(1) | Medium | 😍😍 |
| 090. Subsets II | O(n * 2^n) | O(1) | Medium | recursive(逻辑类似040. Combination Sum II) & 😍😍 iterative（插数） |
| 126. Word Ladder II | O(n * d) | O(d) | Hard |
|
| 131. Palindrome Partitioning | O(n^2) ~ O(2^n) | O(n^2) | Medium | 😍Python Solution |
| 140. Word Break II | O(n * l^2 + n * r) | O(n^2) | Hard | 🎅DFS with Memoization, 没有memoization TLE, ✏️C++ Std:function |
| 212. Word Search II | O(m * n * l) | O(l) | Hard | Suffix Trie (backtracking 是把board 暂时改了, 省去了hashset visited), 难度medium左右, ✏️Python Complex number 表示四个DFS 方向, Dictionary setdefault |
| 526. Beautiful Arrangement | O(n!) | O(n) | Medium | swap, 注意if 条件, 🔍Python Solution |
| 676. Implement Magic Dictionary | O(n) | O(d) | Medium |
|
| 679. 24 Game | O(1) | O(1) | Hard | Complexity: upper bound of 12* 6 * 2 * 4 * 4 * 4 = 9216 possibilities🔍🔍 Python Solution |
| 698. Partition to K Equal Sum Subsets | O(n* 2^n) | O(2^n) | Medium | 😍😍 非常经典题目,
|
| 718. Maximum Length of Repeated Subarray | O(m * n) | O(min(m, n)) | Medium | 🔍 DP |
| 784. Letter Case Permutation | _O(n * 2^n) _ | O(1) | Easy | ✏️Python itertools.product |
| Title | Time | Space | Difficulty | Algorithm Note |
|---|---|---|---|---|
| 1361. Validate Binary Tree Nodes | O(n) | O(n) | Medium | DFS |
DFS 是看有没有path，DP是看有几个path
| Title | Time | Space | Difficulty | Algorithm Note |
|---|---|---|---|---|
| 010. Regular Expression Matching | O(m*n) | O(n) | Hard | 🎅🎅 |
| 044. Wildcard Matching | O(n*m) | O(1) | Hard | dp or greedy (Greedy也是 O(n*m) ) |
| 053. Maximum Subarray | O(n) | O(1) | Easy | 😍 更新res, minsum 的顺序 |
| 062. Unique Paths | O(m * n) | O(m + n) | Medium | |
| 063. Unique Paths II | O(m * n) | O(m + n) | Medium | |
| 064. Minimum Path Sum | O(m * n) | O(m + n) | Medium | |
| 070. Climbing Stairs | O(n) | O(1) | Easy | |
| 072. Edit Distance | O(m*n) | O(m+n) | Hard | 类似的题: |
| 087. Scramble String | O(n^4) | O(n^3) | Hard | 🎅 Memoization |
| 091. Decode Ways | O(n) | O(1) | Medium | 😍😍🎅 similar question: 062. Unique Paths, 070. Climbing Stairs 509. Fibonacci Number |
| 097. Interleaving String | O(m*n) | O(m+n) | Hard | 🎅 DP, DFS, BFS |
| 115. Distinct Subsequences | O(n^2) | O(n) | Hard | 🎅🎅 类似的题: |
| 120. Triangle | O(m*n) | O(n) | Medium | Bottom-up DP |
| 123. Best Time to Buy and Sell Stock III | O(n) | O(n) | Hard | 🎅🎅Why variables order doesn't matter 类似 |
| 132. Palindrome Partitioning II | O(n^2) | O(n) ~O(n) |
Hard | 🎅🎅 类似的题: |
| 139. Word Break | O(n^2) | O(n) | Medium |
|
| 152. Maximum Product Subarray | O(n) | O(1) | Medium | 🎅🎅Prefix Product, Suffix Product |
| 174. Dungeon Game | O(n+m) | O(n)~O(1) | Hard | 🎅 bottom-up DP, Can't start at (0,0) |
| 188. Best Time to Buy and Sell Stock IV | O(k*n) | O(n) | Hard | 类似的题 |
| 198. House Robber | O(n) | O(1) | Easy | Top-down / bottom-up 类似的题: |
| 213. House Robber II | O(n) | O(1) | Medium | 分成两个house rob问题，
类似的题: |
| 221. Maximal Square | O(n^2) | O(n) | Medium | 类似的题: |
| 279. Perfect Squares | O(n * sqrt(n) | O(n) | Medium | Bottom-Up DP, Top-Down DP,BFS. Similar Question |
| 304. Range Sum Query 2D - Immutable | ctor: O(m * n), lookup: O(1) | O(m+n) | Medium | 类似的题: |
| 309. Best Time to Buy and Sell Stock with Cooldown | O(n) | O(1) | Medium | |
| 312. Burst Balloons | O(n^3) | O(n^2) | Hard | Top-Down / Bottom-up, Similar Question: 546. Remove Boxes |
| 322. Coin Change | O(n*k) | O(k) | Medium | Bottom-up, Top-Down, BFS, Similar Question |
| 338. Counting Bits | O(n) | O(n) | Medium | Math 找规律 |
| 357. Count Numbers with Unique Digits | O(n) | O(1) | Medium | 🎅DP, Static DP, backtracking |
| 368. Largest Divisible Subset | O(n^2) | O(n) | Medium | 🎅🎅Key: a < b < c, if c % b = 0 and b % a = 0 Then c % a == 0 类似题： |
| 375. Guess Number Higher or Lower II | O(n^2) | O(n^2) | Medium | 🎅 |
| 377. Combination Sum IV | O(nlogn + n * t) | O(t) | Medium | Similar Question |
| 403. Frog Jump | O(n^2) | O(n^2) | Hard | 经典, TopDown, Bottom-up, BFS |
| 416. Partition Equal Subset Sum | O(n*s) | O(s) | Medium | backtracking / DP |
| 446. Arithmetic Slices II - Subsequence | O(n^2) | O(n*d) | Hard | 🎅 |
| 466. Count The Repetitions | O(s1 * min(s2, n1)) | O(s2) | Hard | 🎅🎅 |
| 467. Unique Substrings in Wraparound String | O(n) | O(1) | Medium | 经典🎅🎅 |
| 472. Concatenated Words | O(n * l^2) | O(l) | hard | suffix Trie |
| 474. Ones and Zeroes | O(s *m * n) | O(s *m * n) | Medium | 经典🎅, Top-Down, Bottom-up |
| 486. Predict the Winner | O(n^2) | O(n) | Medium | 经典🎅🎅, DP解, DFS |
| 514. Freedom Trail | O(k) ~ O(k * r^2) | O(r) | Hard | 经典🎅🎅, Top-Down, Bottom-up |
| 516. Longest Palindromic Subsequence | O(n^2) | O(n) | Medium | 经典🎅, Bottom-up, Top-Down 类似的题: |
| 518. Coin Change 2 | O(n^2) | O(n) | Medium | 经典🎅TopDown, Bottom-up |
| 546. Remove Boxes | O(n^3) ~ O(n^4) | O(n^3) | Hard | 🎅🎅🎅 Top-Down, Bottom-up, Similar Question: 312. Burst Balloons |
| 552. Student Attendance Record II | O(n) | O(1)~O(n) | Hard | 🎅 Derive Relation |
| 576. Out of Boundary Paths | O(N * m * n) | O(m * n) | Medium | DP, DFS, BFS |
| 583. Delete Operation for Two Strings | O(m*n) | O(n) | Medium | Edit Distance without replace 类似题: |
| 600. Non-negative Integers without Consecutive Ones | O(1) | O(1) | Hard | 🎅🎅 Math & Bit |
| 629. K Inverse Pairs Array | O(n*k) | O(k) | Hard | 🎅找规律 |
| 639. Decode Ways II | O(n) | O(1) | Hard | 🎅 巧解 类似的题: 091. Decode Ways |
| 650. 2 Keys Keyboard | O(sqrt(n)) | O(1) | Medium | 🎅 Greedy / DP prime factoring |
| 664. Strange Printer | O(n^3) | O(n^2) | Hard | 🎅🎅 类似的题: |
| 673. Number of Longest Increasing Subsequence | O(n^2) | O(n) | Medium | 💜🎅🎅 |
| 688. Knight Probability in Chessboard | O(k*n^2) | O(k*n^2) ~O(n^2) |
Medium | 💜 Bottom-up, Top-Down |
| 689. Maximum Sum of 3 Non-Overlapping Subarrays | O(n) | O(n) | Hard | 🎅🎅🎅 sliding windows/ DP similar to Stock Purchasing 类似的题 |
| 691. Stickers to Spell Word | O(2^T*S*T) | O(2^T) | Hard | 🎅🎅🎅 |
| 712. Minimum ASCII Delete Sum for Two Strings | O(m*n) | O(m*n) ~O(n) |
Medium | Edit Distance 类似的题: |
| 714. Best Time to Buy and Sell Stock with Transaction Fee | O(n) | O(n) | Medium | |
| 730. Count Different Palindromic Subsequences | O(n^2) | O(n) | Hard | Hard中Hard 🎅🎅🎅 |
| 740. Delete and Earn | O(n) | O(n) | Medium | 类似的题: |
| 741. Cherry Pickup | O(n^3) | O(n^2) | Hard | 🎅🎅🎅 |
| 746. Min Cost Climbing Stairs | O(n) | O(1) | Easy | |
| 764. Largest Plus Sign | O(n^2) | O(n^2) | Medium | Maximal Square, 从左到右，从上到下，从右到左，从下到上,更新最小的count 类似的题: |
| 788. Rotated Digits | O(n)~O(logn) | O(n)~O(logn) | Easy | 🎅🎅🎅 |
| 790. Domino and Tromino Tiling | O(n) | O(n)~O(1) | Medium | 🎅🎅 Math 找规律 |
| 799. Champagne Tower | O(n^2) | O(n^2)~O(n) | Medium | |
| 801. Minimum Swaps To Make Sequences Increasing | O(n) | O(1) | Medium | 🎅 |
| 805. Split Array With Same Average | O(n^4) | O(n^3) | Hard | 💜 🎅🎅🎅 totalSum/n = Asum/k = Bsum/(n-k) |
| 808. Soup Servings | O(1) | O(1) | Medium | |
| 813. Largest Sum of Averages | O(k*n^2) | O(n) | Medium | 💜🎅🎅🎅 |
| 818. Race Car | O(nlogn) | O(n) | Hard | 🎅🎅🎅 |
| 823. Binary Trees With Factors | O(n^2) | O(n) | Medium | 类似题： |
| 837. New 21 Game | O(n) | O(n) | Medium | 🎅🎅🎅 |
| 847. Shortest Path Visiting All Nodes | O(n*2^n) | O(n*2^n) | Hard | BFS/DP🎅 |
| 877. Stone Game | O(n^2) | O(n) | Medium | Strategy |
| 879. Profitable Schemes | O(n*g*p) | O(g*p) | Hard | 💜🎅 |
| 903. Valid Permutations for DI Sequence | O(n^2) | O(n) | Hard | 💜🎅🎅 |
| 920. Number of Music Playlists | O(n*l) | O(n) | Hard | 🎅🎅🎅 |
| 926. Flip String to Monotone Increasing | O(n) | O(n) | Medium | 💜🎅🎅 |
| 931. Minimum Falling Path Sum | O(n^2) | O(n) | Medium | |
| 926. Flip String to Monotone Increasing | O(n) | O(n) | Medium | 💜 |
| 935. Knight Dialer | O(logn) | O(1) | Medium | 💜Citadel真题. Matrix Exponentiation |
| 940. Distinct Subsequences II | O(n) | O(1) | Medium | 💜🎅 |
| 943. Find the Shortest Superstring | O(n^2 * 2^n) | O(n^2) | Medium | 🎅🎅🎅 Travelling Salesman Problem |
| 956. Tallest Billboard | O(n * 3^(n/2)) | O(3^(n/2)) | Hard | 🎅🎅🎅 Knapsnack |
| 960. Delete Columns to Make Sorted III | O(n * l^2) | O(l) | Hard | 🎅类似的题: |
| 975. Odd Even Jump | O(nlogn) | O(n) | Hard | 💜🎅🎅🎅, Mono Stack/BST |
| 983. Minimum Cost For Tickets | O(n) | O(1) | Medium | 💜🎅🎅 Similar Question |
| 1277. Count Square Submatrices with All Ones | O(m*n) | O(1) | Medium | 类似的题: |
| 1387. Sort Integers by The Power Value | O(n) average | O(n) | Medium | nth_element, ✏️✏️C++ Static Variable Python Static Variable |
| 1388. Pizza With 3n Slices | O(n^2) | O(n) | Hard | 😍😍 类似213. House Robber II 和 188. Best Time to Buy and Sell Stock IV |
| 1395. Count Number of Teams | O(n^2) | O(1) | Medium | |
| 1411. Number of Ways to Paint N × 3 Grid | O(logn) | O(1) | Medium | 😍😍 Matrix Exponentiation |
| 1420. Build Array Where You Can Find The Maximum Exactly K Comparisons | O(n*m*k) | O(m*k) | Hard | 🎅 |
| Title | Time | Space | Difficulty | Algorithm Note |
|---|---|---|---|---|
| 146. LRU Cache | O(1) | O(k) | Medium |
|
| 380. Insert Delete GetRandom O(1) | O(1) | O(1) | Medium | 🎅🎅 |
| 1381. Design a Stack With Increment Operation | ctor: O(1) |
|||
| push: O(1) |
||||
| pop: O(1) |
||||
| increment: O(1) | O(n) | Medium | Lazy increment | |
| [1396. Design Underground System](1396 Design Underground System) | ctor: O(1) |
|||
| checkin: O(1) |
||||
| checkout: O(1) |
||||
| getaverage: O(1) | O(n) | Medium |
| Title | Time | Space | Difficulty | Algorithm Note |
|---|---|---|---|---|
| 192 Word Frequency | O(n) | O(k) | Medium | switch column awk, remove whitespace sed |
| 193. Valid Phone Numbers | O(n) | O(1) | Easy | grep |
| 194 Transpose File Shell | O(n^2) | O(n^2) | Medium | paste & cut |
| 195. Tenth Line | O(n) | O(1) | Easy | awk, sed |