Implement Trie (Prefix Tree) 字典树

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

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

设计一个字典树（前缀树），题目中假设字典中包含的字母为a-z, 我们可以设定树中每个节点都有两个属性，一个为TrieNode[]， 另一个为一个布尔变量isLast。TrieNode[] 用来存放insert操作时对应的字符，isLast对应search操作是是否包含这个元素。代码如下：

class TrieNode {
    boolean isLast;
    TrieNode[] trieNode;
    // Initialize your data structure here.
    public TrieNode() {
        isLast = false;
        trieNode = 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(char c : word.toCharArray()) {
            if(cur.trieNode[c - 'a'] == null) {
                cur.trieNode[c - 'a'] = new TrieNode();
            }
            cur = cur.trieNode[c - 'a'];
        }
        cur.isLast = true;
    }

    // Returns if the word is in the trie.
    public boolean search(String word) {
        TrieNode cur = root;
        for(char c : word.toCharArray()) {
            if(cur.trieNode[c - 'a'] == null) 
                return false;
            cur = cur.trieNode[c - 'a'];
        }
        return cur.isLast;
    }

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

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