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 };