Flip Game II -- LeetCode

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。

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