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

Example:
Trie trie = new Trie();

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

Note:
You may assume that all inputs are consist of lowercase letters a-z.
All inputs are guaranteed to be non-empty strings.

难度：medium

题目：
实现Trie树（字典树），插入，搜索和前缀方法。
你可以假定所有的输入都是小写字母，所有输入都是非空(not empty)字符串。

Runtime: 89 ms, faster than 98.80% of Java online submissions for Implement Trie (Prefix Tree).

class Trie {
    private TrieNode root;
    /** Initialize your data structure here. */
    public Trie() {
        root = new TrieNode();
    }
    
    /** Inserts a word into the trie. */
    public void insert(String word) {
        char c = word.charAt(0);
        int i = 0, idx = 0;
        TrieNode ptr = root;
        for (; i < word.length(); i++, ptr = ptr.next[idx]) {
            idx = word.charAt(i) - 'a';
            if (null == ptr.next[idx]) {
                ptr.next[idx] = new TrieNode();
            }
        }
        
        ptr.isWord = true;
    }
    
    /** Returns if the word is in the trie. */
    public boolean search(String word) {
        char c = word.charAt(0);
        int i = 0, idx = 0;
        TrieNode ptr = root;
        for (; i < word.length() - 1; i++, ptr = ptr.next[idx]) {
            idx = word.charAt(i) - 'a';
            if (null == ptr.next[idx]) {
                return false;
            }
        }
        
        TrieNode end = ptr.next[word.charAt(i) - 'a'];
        
        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) {
        int idx = 0, i = 0;
        TrieNode ptr = root;
        for (; i < prefix.length(); i++, ptr = ptr.next[idx]) {
            idx = prefix.charAt(i) - 'a';
            if (null == ptr.next[idx]) {
                return false;
            }
        }
        
        return i >= prefix.length();
    }
    
    public static class TrieNode {
        public boolean isWord;
        public TrieNode[] next = new TrieNode[26];
    }
}

/**
 * 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);
 */