You are playing the following Flip Game with your friend: Given a string that contains only these two characters: + and -, you and your friend take turns to flip two consecutive "++" into "--". The game ends when a person can no longer make a move and therefore the other person will be the winner.

Write a function to determine if the starting player can guarantee a win.

For example, given s = "++++", return true. The starting player can guarantee a win by flipping the middle "++" to become "+--+".

Follow up:
Derive your algorithm‘s runtime complexity.

 1 class Solution { 2 public: 3     bool canWin(string s) { 4         if (s.size() < 2) return false; 5          6         for (int i = 1; i < s.size(); i++) { 7             if (s[i] == ‘+‘ && s[i - 1] == ‘+‘) { 8                 s[i] = s[i - 1] = ‘-‘; 9                 if (!canWin(s)) return true;10                 s[i] = s[i - 1] = ‘+‘;11             }12         }13         return false;14     }15 };

Flip Game II