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

class Trie {

   private class TrieNode {
        Map<Character, TrieNode> children;
        boolean wordEnd;
        public TrieNode() {
            children = new HashMap<Character, TrieNode>();
            wordEnd = false;
        }
    }

    private TrieNode root;

    /** Initialize your data structure here. */
    public Trie() {
        root = new TrieNode();
        root.wordEnd = false;
    }

    /** Inserts a word into the trie. */
    public void insert(String word) {
        TrieNode node = root;
        for (int i = 0; i < word.length(); i++) {
            Character c = new Character(word.charAt(i));
            if (!node.children.containsKey(c)) {
                node.children.put(c, new TrieNode());
            }
            node = node.children.get(c);
        }
        node.wordEnd = true;
    }

    /** Returns if the word is in the trie. */
    public boolean search(String word) {
        TrieNode node = root;
        for (int i = 0; i < word.length(); i++) {
            Character c = new Character(word.charAt(i));
            if (!node.children.containsKey(c)) {
                 return false;
            }
            node = node.children.get(c);
        }
        return node.wordEnd;

    }

    /** Returns if there is any word in the trie that starts with the given prefix. */
    public boolean startsWith(String prefix) {
        TrieNode node = root;
        for (int i = 0; i < prefix.length(); i++) {
            Character c = new Character(prefix.charAt(i));
            if (!node.children.containsKey(c)) {
                 return false;
            }
            node = node.children.get(c);
        }
        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);