Implement a trie with insert, search, and startsWith methods.
Note:
You may assume that all inputs are consist of lowercase letters a-z.

思路：
定义一个节点表示TrieNode，由于题目说明只有a-z，因此子节点用一个26长度的数组即可，此外用一个bool变量表示到当前节点为止是否为一个单词。

public class GJ_TrieTree {
class TrieNode {
    public char c;
    public TrieNode[] children;
    public boolean isWord;

    public TrieNode(char c) {
        this.c = c;
        this.children = new TrieNode[256];
        this.isWord = false;
    }
}

public TrieNode root;

/** Initialize your data structure here. */
public Trie() {
    this.root = new TrieNode('0');
}

/** Inserts a word into the trie. */
public void insert(String word) {
    TrieNode dummy = this.root;
    for (int i = 0; i < word.length(); i++) {
        char c = word.charAt(i);
        if (dummy.children[c] == null) {
            dummy.children[c] = new TrieNode(c);
        }
        dummy = dummy.children[c];
    }
    dummy.isWord = true;
}

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

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

private TrieNode findHelper(String str) {
    TrieNode dummy = this.root;

    for (int i = 0; i < str.length(); i++) {
        char c = str.charAt(i);
        if (dummy.children[c] == null) {
            return null;
        }
        dummy = dummy.children[c];
    }

    return dummy;
}

}