1 class Solution {
 2 public:
 3     string reverseString(string s) {
 4         stack<char> tem;
 5         string result;
 6         for ( auto c : s)
 7             tem.push(c);
 8         while (!tem.empty()){
 9             result += tem.top();
10             tem.pop();
11         }
12         return result;
13     }
14 };

 1 class Solution {
 2 public:
 3     string reverseString(string s) {
 4         if (s.size() < 2)
 5             return s;
 6         int l = 0, r = static_cast<int>(s.size() - 1);
 7         while (l < r)
 8         {
 9                 swap(s[l++], s[r--]);
10         }
11         return s;
12     }
13 };