【LeetCode 208】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.

思路：

构建一个简单的字典树，要求实现插入、查找、查找前缀三个操作。这里使用map实现对子树的构建（一方面方便查找，另一方面是懒 - -！），但是效率并不是太高，LeetCode上提交后跑了147ms，不是很理想，不过不影响对字典树这种树形结构的理解。

 class TrieNode {
 public:
     // Initialize your data structure here.
     TrieNode(char c):ch(c), isEnd(false){}

     map<char, TrieNode *> childNode;

     char ch;
     bool isEnd;
 };

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

     // Inserts a word into the trie.
     void insert(string s) {

         TrieNode *t = root;

         //从root遍历待插入字符串，若c存在则继续向下遍历，反之则新建一个分支
         for(int i = ; i < s.length(); i++)
         {
             char c = s[i];

             map<char, TrieNode*>::iterator it = t->childNode.find(c);
             if(it == t->childNode.end())
             {
                 TrieNode *newNode = new TrieNode(c);
                 t->childNode.insert(make_pair(c, newNode));
                 t = newNode;
             }
             else
             {
                 t = t->childNode[c];
             }
         }
         t->isEnd = true;
     }

     // Returns if the word is in the trie.
     bool search(string key) {
         TrieNode *s = root;

         //从root遍历待查找字符串，若c存在则向下遍历，反之则不存在
         for(int i = ; i < key.length(); i++)
         {
             char c = key[i];

             map<char, TrieNode*>::iterator it = s->childNode.find(c);
             if(it == s->childNode.end())
                 return false;
             else
                 s = s->childNode[c];
         }
         //返回最后一个结点的状态，判断是否为一个word
         return s->isEnd;
     }

     // Returns if there is any word in the trie
     // that starts with the given prefix.
     bool startsWith(string prefix) {
         TrieNode *s = root;

         //与search一致
         for(int i = ; i < prefix.length(); i++)
         {
             char c = prefix[i];

             map<char, TrieNode*>::iterator it = s->childNode.find(c);

             if(it == s->childNode.end())
                 return false;
             else
                 s = s->childNode[c];
         }
         //只要找到即返回true
         return true;
     }

 private:
     TrieNode* root;
 };