344.反转字符串

代码：

 1 void reverseString(vector<char>& s) {
 2 
 3     int i = 0;
 4     int j = s.size() - 1;
 5     while (i < j)
 6     {
 7         char mid = s[i];
 8         s[i] = s[j];
 9         s[j] = mid;
10 
11         i++;
12         j--;
13     }
14 }