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.

由于用的是 26位字母的array， 所以在trieNode 上甚至不用存val。

class TrieNode {
    // Initialize your data structure here.
    Boolean isWord;
    TrieNode[] children;
    
    public TrieNode(){
        this.isWord = false;
        children = new TrieNode[26];
    }
}

public class Trie {
    private TrieNode root;

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

    // Inserts a word into the trie.
    public void insert(String word) {
        TrieNode cur = root;
        for(int i = 0; i < word.length(); i ++){
            Character c = word.charAt(i);
            if(cur.children[c - 'a'] == null){
                TrieNode tmp = new TrieNode();
                cur.children[c - 'a'] = tmp;
            }
            cur = cur.children[c - 'a'];
        }
        cur.isWord = true;
    }
    
    private TrieNode findNode(String word){
        TrieNode cur = root;
        for(int i = 0; i < word.length(); i ++){
            Character c = word.charAt(i);
            if(cur.children[c - 'a'] == null){
                return null;
            }
            cur = cur.children[c - 'a'];
        }
        return cur;
    }
    
    // Returns if the word is in the trie.
    public boolean search(String word) {
        TrieNode end = findNode(word);
        return end != null && end.isWord;
    }

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

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