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 twoconsecutive"++"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 boolean canWin(String s) {
        String temp = "";
        for (int i = 1; i < s.length(); i++){
            if (s.charAt(i) == '+' && s.charAt(i - 1) == '+'){
                String cur = s.substring(0, i - 1) + "--" + s.substring(i + 1);
                if (!canWin(cur)) return true;
            }
        }

        return false;
    }
}