实现前缀树

题目描述：
实现一个 Trie (前缀树)，包含 insert, search, 和 startsWith 这三个操作。

示例：
Trie trie = new Trie();

trie.insert("apple");
trie.search("apple"); // 返回 true
trie.search("app"); // 返回 false
trie.startsWith("app"); // 返回 true
trie.insert("app");
trie.search("app"); // 返回 true

Java代码：

class Trie {

    class TrieNode{
        private boolean isEnd;
        TrieNode[] next;

        public TrieNode() {
            isEnd = false;
            next = new TrieNode[26];
        }
    }

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

/**
 * 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);
 */