[LeetCode] Implement Trie (Prefix Tree)

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

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

实现字典树，没啥好说的。

 class TrieNode {
 public:
     // Initialize your data structure here.
     TrieNode *ch[];
     bool isKey;
     TrieNode() : isKey(false) {
         for (auto &a : ch) a = NULL;
     }
 };

 class Trie {
 public:
     Trie() {
         root = new TrieNode();
     }

     // Inserts a word into the trie.
     void insert(string s) {
         TrieNode *p = root;
         for (auto &a : s) {
             int i = a - 'a';
             if (p->ch[i] == NULL) p->ch[i] = new TrieNode();
             p = p->ch[i];
         }
         p->isKey = true;
     }

     // Returns if the word is in the trie.
     bool search(string key) {
         TrieNode *p = root;
         for (auto &a : key) {
             int i = a - 'a';
             if (p->ch[i] == NULL) return false;
             p = p->ch[i];
         }
         return p->isKey;
     }

     // Returns if there is any word in the trie
     // that starts with the given prefix.
     bool startsWith(string prefix) {
         TrieNode *p = root;
         for (auto &a : prefix) {
             int i = a - 'a';
             if (p->ch[i] == NULL) return false;
             p = p->ch[i];
         }
         return true;
     }

 private:
     TrieNode* root;
 };

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