[LeetCode][JavaScript]Implement Trie (Prefix Tree)

Implement Trie (Prefix Tree)

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

https://leetcode.com/problems/implement-trie-prefix-tree/

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实现字典树，每个节点至多有26个子孙，代表26个字母。

每个节点都有三个属性，key, isWord以及字典(哈希表，提高访问速度，也可以用数组)，因为JS可以扩展实例化的对象，直接用下标访问对象，不需要再建一个哈希表了。

root节点没有key和isWord，剩下的节点key记录该节点的值。

isWord：比如单词为"tree"，"tre" -> 不是单词， "tree" -> 是单词。

构造完字典后，比如要查找"tree"，只需要这样访问：root['t']['r']['e']['e'].isWord，任意一步拿不到就说明单词不在字典中。

/**
 * @constructor
 * Initialize your data structure here.
 */
var TrieNode = function(key) {
    return {
        key : key,  
        isWord : false
    };
};

var Trie = function() {
    this.root = TrieNode('root');
};

/**
 * @param {string} word
 * @return {void}
 * Inserts a word into the trie.
 */
Trie.prototype.insert = function(word) {
    var tree = this.root, i, curr;
    for(i = 0; i < word.length; i++){
        curr = word[i];
        if(!tree[curr]){
            tree[curr] = new TrieNode(curr);
        }
        tree = tree[curr];
    }
    tree.isWord = true;
};

/**
 * @param {string} word
 * @return {boolean}
 * Returns if the word is in the trie.
 */
Trie.prototype.search = function(word) {
    var tree = this.root;
    for(var i = 0; i < word.length; i++){
        if(!tree[word[i]]){
            return false;
        }
        tree = tree[word[i]];
    }
    return tree.isWord ? true : false;
};

/**
 * @param {string} prefix
 * @return {boolean}
 * Returns if there is any word in the trie
 * that starts with the given prefix.
 */
Trie.prototype.startsWith = function(prefix) {
    var tree = this.root;
    for(var i = 0; i < prefix.length; i++){
        if(!tree[prefix[i]]){
            return false;
        }
        tree = tree[prefix[i]];
    }
    return true;
};