LeetCode #294. Flip Game II

题目描述：

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.

Example:

Input: s = "++++"

Output: true 
Explanation: The starting player can guarantee a win by flipping the middle "++" to become "+--+".

Follow up:
Derive your algorithm's runtime complexity.

class Solution {
public:
    bool canWin(string s) {
        for (int i=1;i<s.size();i++) 
        {
            if (s[i]=='+'&&s[i-1]=='+'&&!canWin(s.substr(0,i-1)+"--"+s.substr(i+1)))
                return true;
        }
        return false;
    }
};