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.

思路：递归求解。

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

Flip Game II -- LeetCode