Flip Game II

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 };