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.

class Solution {
public:
    bool canWin(string s) {
        if (s.size() < 2) return false;
        
        for (int i = 1; i < s.size(); i++) {
            if (s[i] == '+' && s[i - 1] == '+') {
                s[i] = s[i - 1] = '-';
                if (!canWin(s)) return true;
                s[i] = s[i - 1] = '+';
            }
        }
        return false;
    }
};