Although the Notebook contains a bunch of problem solutions, we demand more.
The notebook aims at helping us become skilled at solving problems using Data structures, Algorithms and tricks properly.
Most problems will be hard with hands dirty if one uses "Brute Force",
but if we work on them with proper Data structures, Algorithms and tricks,
the life will become quite easy as well as with a high quality yield.
Instead of through hard working on problems again and again,
our motivation is to develop problem-solving skills wisely
by applying and deveoping meta-skills such as abstracting, grouping, finding properties, connecting, comparing, evaluating, etc..
(Sounds like working with Category Theory, doesn't it?)
Parallelly, there will be a collection of common procedures and steps, guidelines and tips available.
Furthermore, through this project,
we hope to collect and develop (usually by abstracting) strategy
for developing meta-skills
useful in developing problem-solving skills.
They can be viewed layer by layer vertically.
And horizontally, we need to supply common procedures and steps, guidelines and tips by abstracting.
These abilities and the strategy are common in other domains of materials.
The higher demand will motivate us through the journey.
How to use LeetCode
Therefore,
LeetCode problems are no more isolated individual problems,
but a repository of problems with hidden connections of similarity and variance.
We need to manage time wisely instead of hacking on everyone starting from problem #1 which is frustrating.
Note LeetCode is NOT the place for learning fundamental concepts, we should do that before starting LeetCode.
The repository are trainings for developing individual skills
and also for applying them flexibly and logically.
Moreover, the problems are putting together for abstracting the common procedures and steps for solving problems.
Although it will be slower at the beginning,
but it will become much faster at the end with additional gain of higher level abilities.
Map
The knowledges are organized in groups of "Analysis-Solutions", "Algorithms" and "Data-structures".
More than a structure,
with time passing, we should have a virtual map in our brain for contents as nodes and connections as pathes.
That is, LeetCode problems are no more in a linearly ordered sequence,
but a graph of nodes and connections.
Schedule
One month on "Easy" questions:
They provide us the chance for understanding and getting familiar with concepts and properties.
They are for developing individual basic skills and tricks for the future.
They are the starting point of the journey for expanding our power.
But we do not have to go through all of them,
since many of them are not worth spending much efforts (See Tips).
Therefore we start with questions for fundamental concepts and skills.
And then make sure to cover all "Top 100 Liked Questions" and "Top 100 Interview Questions".
At this time, we focus on implementation skills.
Then one "Medium-Hard" problem per day:
They are essential problems closer to real daily problems.
They are more complexed problems composed of lower level problems with new restrictions,
which require us to play with a bunch of basic building blocks flexibly and logically.
Make sure to cover all "Top 100 Liked Questions" and "Top 100 Interview Questions".
At this time, we focus on solutions skills.
Time Management
Only spend less than 20-30 minutes for generating a solution without implementation.
See "How to come up with own solutions".
On one hand,
it trains us to get used to the real interview scene.
On the other hand,
remember our goal is to get the problem-solving ability through the training.
Our time is valuable, we have to spend our time wisely.
We do not have to beat every question by our own during the training.
No one knows everything when he is growing.
The failure guides us to the next success by showing what knowledge do we need to make up,
and what skills do we need to spend our time for.
See "How to check with other solutions".
How to come up with own solutions
Grab a piece of white paper.
90-10 stratege: 90% time for thinking, 10% time for coding.
Read the description carefully.
Find and write down input-output-functionality, know what is the data structure for input.
Find and write down restrictions.
Abstract tasks and information to higher level as a conceptuality.
Determine which group it belongs to.
Connect to Data Structures, Algorithms and tricks commonly used for the group and estimate if they help to make the tasks easier.
Manipulate this optional solution with a general example to make sure it works.
Find other solutions and repeat above steps, and then estimate their time / space complexity.
Pick one solution and decompose the tasks into sub-tasks according to your chosen tools and mark them down.
Move on to "How to implement your own solutions".
How to implement your own solutions
Never start before coming up with own solutions.
Open a notebook to write it down. It is where we make our hands dirty dealing with details.
Bottom-up refinement with an example.
Start with output or return, place them at different locations.
Around return output to add code blocks for it and comment briefly for how to come up with the return.
Structure the blocks by cases or loops and comment briefly for each one.
Add initializations.
Add guards.
Double check with the margin cases.
Verify by drawing test cases and manipulating them with our code. Modify the code to handle the error.
Use compiler knowledge on codes for possible optimization.
Optimize (make concise) our codes by combining redundant, but sometimes it reduces readibility.
Move on to "After implementation".
After implementation
Copy it to LeetCode and submit.
Record bugs you couldn't find and do reasoning.
Check with otherones answers for improvements.
Move on to "How to check with other solutions".
How to check with other solutions
Benefit to check with standard solutions and also those in "Discuss" (click "Most Votes" and only for <=2 pages).
Use "Debugger" to understand the solution step by step if necessary.
Abstract their tricks and find out how to come up with the tricks from the questions, then extend our tricks set for this topic.
Evaluate and compare time / space complexity of all solutions.
Mark the problem number down and review it in the near future.
Tips
Use filters. E.g., pick "Top 100 Liked Questions".
Do not work on the problems with 👎 >> 👍. It doesn't mean the problem is "bad", but it is too easy if you have worked out "T-I".
Only view discussion with 👍 >> 👎.
Do not connect the problem to a particular problem, but to a general category, because we have limited memory.
Optimization and conciseness are different.
Understand the relationship of Data Structures, i.e., Hash Table with don't care values, then use Set.
Keep in mind the operations can be arranged in different structures, by trade off between readibility and conciseness.
Conciseness might not always help optimization (which is by estimating complexity and mind-compiling), but reduce readability.
We will be skilled at conciseness automatically with the time going, but focus on correctness and time / space complexity.
Do not RUSH! The faster is the slower; The slower is the faster.
Instead of hacking on solving problems, it benefits from reviewing sloved problems as well as other people's solutions.
Content Table
Note we supply solutions as much as possible for inspiration,
but please estimate if it satisfies the question restrictions.
Abbreviations:
"I" stands for Implementations are supplied, which take the training for coding skills
"A" stands for Analysis are supplied, which take the training for coming up solutions, and also the overview of all solutions
"N" stands for no work, either because the analysis and implementation are straight forward, or a slightly alternate to one already seen
"T-I" stands "Top 100 Interview Questions"
"T-100" stands "Top 100 Liked Questions"
"E" stands for "Building blocks", because they are from LeetCode category "Easy"
"E+" stands for "Requiring changes on Building blocks", also from LeetCode category "Easy"
"BFS" stands for "Breadth-First Search"
"BM" stands for "Bit Manipulation"
"BS" stands for "Binary Search"
"DC" stands for "Divide and Conquer"
"DP" stands for "Dynamic Programming"
"DFS" stands for "Depth-First Search"
"GR" stands for "Greedy"
"HT" stands for "Hash Tables"
"LI" stands for "List"
"REC" stands for "Recursion"
"SO" stands for "Sorting"
"TP" stands for "Two Pointers"
"* " stands for "Incomplete"
"⭐" stands for fundamental tasks from LeetCode category "Easy"
"⭐⭐" stands for fundamental tasks from LeetCode category "Easy", but with a little bit change on "⭐"
"🌟" stands for fundamental tasks from LeetCode category "Medium"
Array
#
Title
Difficulty
Level
Algorithms
Related
👍👎
Solutions
Lists
0001
Two Sum
Easy
⭐
HT
>>
N
T-I
0026
Remove Duplicates from Sorted Array
Easy
⭐⭐
TP
27
<<
AI
T-I
0053
Maximum Subarray
Easy
⭐
DC, DP, GR
121
>>
N
T-I
0075
Sort Colors
Medium
🌟
TP
>>
I
T-I
0088
Merge Sorted Array
Easy
⭐
TP
21
<
A
T-I
0121
Best Time to Buy and Sell Stock
Easy
⭐⭐
DC, DP, GR
53
>>
A
T-I
0122
Best Time to Buy and Sell Stock II
Easy
⭐
GR
>>
I
T-I
0136
Single Number
Easy
⭐
HT, SO, BM, MA
242
>>
A
T-I
0169
Majority Element
Easy
⭐
HT, SO, DC, REC
>>
N
T-I
0198
House Robber
Medium
🌟
DP
746
>>
I
T-I
0217
Contains Duplicate
Easy
⭐
SO, Set, HT
>>
AI
T-I
0283
Move Zeroes
Easy
⭐⭐
TP
27
>>
A
T-I
0322
Coin Change
Medium
🌟
DP
>>
I
T-I
0350
Intersection of Two Arrays II
Easy
⭐
TP, BS, HT, SO
>>
N
T-I
String
#
Title
Difficulty
Level
Algorithms
Related
👍👎
Solutions
Lists
0014*
Longest Common Prefix
Easy
⭐
DC, BS
>>
A
T-I
0020
Valid Parentheses
Easy
⭐
STK
>>
N
T-I
0028
Implement strStr()
Easy
⭐
TP
=
A
T-I
0038
Count and Say
Easy
<
N
T-I
0125
Valid Palindrome
Easy
⭐⭐
TP
304,709
<<
N
T-I
0242
Valid Anagram
Easy
⭐⭐
SO
217
>>
AI
T-I
0344
Reverse String
Easy
⭐
TP, REC
27
>>
AI
T-I
0387
First Unique Character in a String
Easy
⭐
HT, Set
>>
N
T-I
LinkedList
#
Title
Difficulty
Level
Algorithms
Related
👍👎
Solutions
Lists
0002
Add Two Numbers
Medium
🌟
>>
I
T-I
0019
Odd Even Linked List
Medium
🌟
TP
>>
N
T-I
0021
Merge Two Sorted Lists
Easy
⭐
TP, REC
88
>>
I
T-I
0141
Linked List Cycle
Easy
⭐
TP, Set
>>
AI
T-I
0160
Intersection of Two Linked Lists
Easy
⭐
TP, Set
>>
A
T-I
0206
Reverse Linked List
Easy
⭐
REC
>>
AI
T-I
0234*
Palindrome Linked List
Easy
⭐⭐
TP, REC
206, 876
>>
A
T-I
0237
Delete Node in a Linked List
Easy
⭐
<<
N
T-I
0328
Odd Even Linked List
Medium
🌟
TP
>>
A
T-I
Math & Stack
#
Title
Difficulty
Level
Algorithms
Related
👍👎
Solutions
Lists
0007
Reverse Integer
Easy
⭐
326
<
N
T-I
0013
Roman to Integer
Easy
<
N
T-I
0069
Sqrt(x)
Easy
⭐
BS, BM
367
<
N
T-I
0070
Climbing Stairs
Easy
⭐
DC, REC
509
>>
AI
T-I
0171
Excel Sheet Column Number
Easy
⭐
>>
N
T-I
0172
Factorial Trailing Zeroes
Easy
⭐
=
N
T-I
0202
Happy Number
Easy
⭐
HT, Set
>>
I
T-I
0204
Count Primes
Easy
⭐
HT
>>
N
T-I
0268
Missing Number
Easy
⭐
BM, SO, HT
=
N
T-I
0326
Power of Three
Easy
⭐
<<
N
T-I
Tree
#
Title
Difficulty
Level
Algorithms
Related
👍👎
Solutions
Lists
0101
Symmetric Tree
Easy
⭐
BFS, DFS
>>
T-I
0104
Maximum Depth of Binary Tree
Easy
⭐
BFS, DFS, REC
>>
T-I
0108
Convert Sorted Array to Binary Search Tree
Easy
⭐
BFS, DFS
>>
T-I
Non Top 100 Interview Questions
Array
#
Title
Difficulty
Level
Algorithms
Related
👍👎
Solutions
Lists
0027
Remove Element
Easy
⭐
TP
<<
A
0035
Search Insert Position
Easy
⭐
BS, REC
704
>>
N
0080
Remove Duplicates from Sorted Array II
Medium
🌟
TP
26
>>
AI
0167
Two Sum II - Input array is sorted
Easy
⭐⭐
TP, BS, HT
1
>>
I
0349
Intersection of Two Arrays
Easy
TO, BS, SO, Set
350
<<
N
0448
Find All Numbers Disappeared in an Array
Easy
>>
0704
Binary Search
Easy
⭐
BS, REC
35
>>
N
0724
Find Pivot Index
Easy
>>
0905
Sort Array By Parity
Easy
>>
String
#
Title
Difficulty
Level
Algorithms
Related
👍👎
Solutions
Lists
0205
Isomorphic Strings
Easy
>>
0290
Word Pattern
Easy
>>
0557
Reverse Words in a String III
Easy
⭐
>>
N
0771
Jewels and Stones
Easy
>>
0844
Backspace String Compare
Easy
⭐
TP, STK
>>
N
1002
Find Common Characters
Easy
HT
>>
N
1047
Remove All Adjacent Duplicates In String
Easy
⭐
STK
>>
LinkedList
#
Title
Difficulty
Level
Algorithms
Related
👍👎
Solutions
Lists
0083
Remove Duplicates from Sorted List
Easy
⭐⭐
REC
203, 26
>>
I
0203
Remove Linked List Elements
Easy
⭐
REC
>>
AI
0876
Middle of the Linked List
Easy
⭐
TP, LI
>>
AI
Math & Stack
#
Title
Difficulty
Level
Algorithms
Related
👍👎
Solutions
Lists
0118
Pascal's Triangle
Easy
>>
0119
Pascal's Triangle II
Easy
>>
0155
Min Stack
Easy
⭐
STK
>>
0190
Reverse Bits
Easy
⭐
TP, BM
>>
N
0191
Number of 1 Bits
Easy
BM
>
0232
Implement Queue using Stacks
Easy
⭐
STK, QU
>>
0367
Valid Perfect Square
Easy
⭐
BS
69
>>
N
0374
Guess Number Higher or Lower
Easy
⭐
BS
704
<
N
0389
Find the Difference
Easy
⭐
BM, HT
>>
N
0509
Fibonacci Number
Easy
⭐
REC, DP
70
>
N
0566
Reshape the Matrix
Easy
>
0674
Longest Continuous Increasing Subsequence
Easy
⭐
TP, DP
>>
N
0746
Min Cost Climbing Stairs
Easy
⭐⭐
DP
70
>>
I
0766
Toeplitz Matrix
Easy
>>
0832
Flipping an Image
Easy
>>
0867
Transpose Matrix
Easy
=
Tree
#
Title
Difficulty
Level
Algorithms
Related
👍👎
Solutions
Lists
0100
Same Tree
Easy
>>
0107
Binary Tree Level Order Traversal II
Easy
>>
0112
Path Sum
Easy
>>
0226
Invert Binary Treee
Easy
>>
0404
Sum of Left Leaves
Easy
>>
0559
Maximum Depth of N-ary Tree
Easy
>>
0572
Subtree of Another Tree
Easy
>>
0700
Search in a Binary Search Tree
Easy
>>
0872
Leaf-Similar Trees
Easy
>>
0938
Range Sum of BST
Easy
REC
>>
0993
Cousins in Binary Tree
Easy
>>
Done before (NOT 👍 >> 👎. Easy if you have worked out "T-I".)
#
Title
Difficulty
Level
Algorithms
Related
👍👎
Solutions
Lists
0058
Length of Last Word
Easy
⭐
<<
N
0066
Plus One
Easy
⭐
<
N
0219
Contains Duplicate II
Easy
=
0345
Reverse Vowels of a String
Easy
<<
0383
Ransom Note
Easy
>
0412
Fizz Buzz
Easy
<
0414
Third Maximum Number
Easy
<<
0434
Number of Segments in a String
Easy
<<
0485
Max Consecutive Ones
Easy
>
0520
Detect Capital
Easy
>
0521
Longest Uncommon Subsequence I
Easy
<<
0541
Reverse String II
Easy
<<
0551
Student Attendance Record I
Easy
<
0561
Array Partition I
Easy
=
0589
N-ary Tree Preorder Traversal
Easy
>
0605
Can Place Flowers
Easy
>
0657
Robot Return to Origin
Easy
>
0697
Degree of an Array
Easy
=
0705
Design HashSet
Easy
>
0706
Design HashMap
Easy
>>
0709
To Lower Case
Easy
⭐
<<
N
0717
1-bit and 2-bit Characters
Easy
<<
0804
Unique Morse Code Words
Easy
=
0819
Most Common Word
Easy
<<
0824
Goat Latin
Easy
<<
0830
Positions of Large Groups
Easy
>
0859
Buddy Strings
Easy
>
0888
Fair Candy Swap
Easy
>
0893
Groups of Special-Equivalent Strings
Easy
<<
0896
Monotonic Array
Easy
>
0965
Univalued Binary Tree
Easy
>
1160
Find Words That Can Be Formed by Characters
Easy
>
1189
Maximum Number of Balloons
Easy
>
1207
Unique Number of Occurrences
Easy
>
Todo (for up >> down and up > 1000) For practicing topics if needed
#
Title
Difficulty
Level
Algorithms
Related
👍👎
Solutions
Lists
0009
Palindrome Number
Easy
>>
0067
Add Binary
Easy
>>
0168
Excel Sheet Column Title
Easy
>>
0175
Combine Two Tables
Easy
>>
0231
Power of Two
Easy
>>
0235
Lowest Common Ancestor of a BST
Easy
>>
0278
First Bad Version
Easy
>>
0392
Is Subsequence
Easy
>>
0409
Longest Palindrome
Easy
>>
0415
Add Strings
Easy
>>
0459
Repeated Substring Pattern
Easy
>>
0461
Hamming Distance
Easy
>>
0463
Island Perimeter
Easy
>>
0476
Number Complement
Easy
>>
0501
Find Mode in Binary Search Tree
Easy
>>
0530
Minimum Absolute Difference in BST
Easy
>>
0543
Diameter of Binary Tree
Easy
>>
0617
Merge Two Binary Trees
Easy
>>
0628
Maximum Product of Three Numbers
Easy
>>
0637
Average of Levels in Binary Tree
Easy
>>
0665
Non-decreasing Array
Easy
>>
0653
Two Sum IV - Input is a BST
Easy
>>
0669
Trim a Binary Search Tree
Easy
>>
0680
Valid Palindrome II
Easy
>>
0696
Count Binary Substrings
Easy
>>
0703
Kth Largest Element in a Stream
Easy
>>
0733
Flood Fill
Easy
>>
0796
Rotate String
Easy
>>
0821
Shortest Distance to a Character
Easy
>>
0929
Unique Email Addresses
Easy
>>
0942
DI String Match
Easy
>>
0953
Verifying an Alien Dictionary
Easy
>>
0977
Squares of a Sorted Array
Easy
>>
0997
Find the Town Judge
Easy
>>
1046
Last Stone Weight
Easy
>>
1154
Day of the Year
Easy
>>
1365
How Many Numbers Are Smaller Than the Current Number
Easy
>>
About
More than a bunch of LeetCode problem solutions, the cookbook focus on the strategies for spending your time wisely to improve your ability.