LeetCode 208. Implement Trie (Prefix Tree)
Woodyiiiiiii opened this issue · comments
Implement a trie with insert
, search
, and startsWith
methods.
Example:
Trie trie = new Trie();
trie.insert("apple");
trie.search("apple"); // returns true
trie.search("app"); // returns false
trie.startsWith("app"); // returns true
trie.insert("app");
trie.search("app"); // returns true
Note:
- You may assume that all inputs are consist of lowercase letters
a-z
. - All inputs are guaranteed to be non-empty strings.
这道题是字典树(Trie) 的入门题目,又称前缀树(prefix tree)或者单词查找树。字典树是一个用来检索字符串(验证一个字符串是否在一堆字符串中或者是前缀)、字符串最长公共前缀等功能的数据结构。
Trie是一个用空间换取时间的应用,插入、查找的时间复杂度均为O(N),其中N为字符串长度,而 空间复杂度是26^n级别的,非常庞大(可采用双数组实现改善)。
详细资料可以观看参考资料。LeetCode中还有关于字典树的类似题目还有No.211题,其中难点在于如何解决正则表达式中的“.”表达,使用了典型的DFS方法,参数表示下标而不再是字符串我没有想出来,太蠢了。
具体如何实现看如下代码:
class Trie {
TreeNode root;
class TreeNode {
TreeNode[] nodes = new TreeNode[26];
boolean end = false;
}
/** Initialize your data structure here. */
public Trie() {
root = new TreeNode();
}
/** Inserts a word into the trie. */
public void insert(String word) {
TreeNode tmp = root;
for (int i = 0; i < word.length(); ++i) {
int idx = word.charAt(i) - 'a';
if (tmp.nodes[idx] == null) {
tmp.nodes[idx] = new TreeNode();
}
tmp = tmp.nodes[idx];
}
tmp.end = true;
}
private TreeNode searchNode(String word) {
TreeNode tmp = root;
for (int i = 0; i < word.length(); ++i) {
int idx = word.charAt(i) - 'a';
if (tmp.nodes[idx] == null) {
return null;
}
tmp = tmp.nodes[idx];
}
return tmp;
}
/** Returns if the word is in the trie. */
public boolean search(String word) {
TreeNode tmp = searchNode(word);
return tmp == null ? false : tmp.end;
}
/** Returns if there is any word in the trie that starts with the given prefix. */
public boolean startsWith(String prefix) {
return searchNode(prefix) == null ? false : true;
}
}
/**
* Your Trie object will be instantiated and called as such:
* Trie obj = new Trie();
* obj.insert(word);
* boolean param_2 = obj.search(word);
* boolean param_3 = obj.startsWith(prefix);
*/
类似题目:
参考资料: