[LeetCode] 208. Implement Trie (Prefix Tree) ☆☆☆

Implement a trie with

insert

,

search

, and

startsWith

methods.

Note:
You may assume that all inputs are consist of lowercase letters

a-z

.

解法：

　　Trie（字典树）的知识参见：数据结构之Trie树 和 [LeetCode] Implement Trie (Prefix Tree) 实现字典树(前缀树)。

　　可以采用数组和哈希表的方式实现，代码分别如下：

public class Trie {
private TrieNode root;

/** Initialize your data structure here. */
public Trie() {
root = new TrieNode();
}

/** Inserts a word into the trie. */
public void insert(String word) {
root.insert(word, 0);
}

/** Returns if the word is in the trie. */
public boolean search(String word) {
TrieNode result = root.search(word, 0);
return result != null && result.getIsWord();
}

/** Returns if there is any word in the trie that starts with the given prefix. */
public boolean startsWith(String prefix) {
TrieNode result = root.search(prefix, 0);
return result != null;
}
}

class TrieNode {
private TrieNode[] children;
private boolean isWord;

public TrieNode() {
children = new TrieNode[26];
isWord = false;
}

public void insert(String word, int index) {
// 如果所有字符都已插入，需要将最后一个字符节点的isWord改为true
if (index == word.length()) {
this.isWord = true;
return;
}
// 如果不存在该字符，在对应位置新建节点
int n = word.charAt(index) - 'a';
if (children
== null) {
children
= new TrieNode();
}
// 继续下一字符
children
.insert(word, index + 1);
}

// 由于Trie中既要求实现search，又要求实现startsWith，为了方便，
// 此处直接返回搜索结果的TrieNode，交由Trie去判断。
public TrieNode search(String word, int index) {
if (index == word.length()) {
return this;
}
int n = word.charAt(index) - 'a';
if (children
== null) {
return null;
}
return children
.search(word, index + 1);
}

public boolean getIsWord() {
return this.isWord;
}
}

/**
* 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);
*/

public class Trie {
private TrieNode root;

/** Initialize your data structure here. */
public Trie() {
root = new TrieNode();
}

/** Inserts a word into the trie. */
public void insert(String word) {
TrieNode curr = root;
for (int i = 0; i < word.length(); i++) {
char letter = word.charAt(i);
if (!curr.children.containsKey(letter)) {
curr.children.put(letter, new TrieNode());
}
curr = curr.children.get(letter);
}
curr.isWord = true;
}

/** Returns if the word is in the trie. */
public boolean search(String word) {
TrieNode result = find(word);
return result != null && result.isWord;
}

/** Returns if there is any word in the trie that starts with the given prefix. */
public boolean startsWith(String prefix) {
TrieNode result = find(prefix);
return result != null;
}

public TrieNode find(String word) {
TrieNode curr = root;
for (int i = 0; i < word.length(); i++) {
char letter = word.charAt(i);
if (!curr.children.containsKey(letter)) {
return null;
}
curr = curr.children.get(letter);
}
return curr;
}
}

class TrieNode {
HashMap<Character, TrieNode> children;
boolean isWord;

public TrieNode() {
children = new HashMap<>();
isWord = false;
}
}

/**
* 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);
*/