Implement Trie (Prefix Tree) ——LeetCode

Implement a trie with insert, search, and startsWith methods.

Note:
You may assume that all inputs are consist of lowercase letters a-z.

实现一个字典树。

好久不做题，没感觉啊，TreeNode用一个布尔变量表示是否是一个合法单词的结尾即可，一开始还用cnt来计数，search的时候比较麻烦。

class TrieNode {
    // Initialize your data structure here.
     TrieNode [] next;
    boolean valid;
    public TrieNode() {
        next = new TrieNode[26];
        valid=false;
    }
}

public class Trie {
    private TrieNode root;

    public Trie() {
        root = new TrieNode();
    }

    // Inserts a word into the trie.
    public void insert(String word) {
        TrieNode ptr = root;
        for(char c:word.toCharArray()){
            if(ptr.next[c-'a']==null){
                ptr.next[c-'a'] = new TrieNode();
            }
            ptr=ptr.next[c-'a'];
        }
        ptr.valid=true;
    }

    // Returns if the word is in the trie.
    public boolean search(String word) {
        int last = 0;
        TrieNode ptr = root;
        for(char c:word.toCharArray()){
            if(ptr==null||ptr.next[c-'a']==null){
                return false;
            }
            ptr=ptr.next[c-'a'];
        }
        return ptr.valid;
    }

    // Returns if there is any word in the trie
    // that starts with the given prefix.
    public boolean startsWith(String prefix) {
        TrieNode ptr = root;
        for(char c:prefix.toCharArray()){
            if(ptr==null||ptr.next[c-'a']==null){
                return false;
            }
            ptr=ptr.next[c-'a'];
        }
        return true;
    }
}

// Your Trie object will be instantiated and called as such:
// Trie trie = new Trie();
// trie.insert("somestring");
// trie.search("key");