Leetcode: 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.

 1 public class Solution {
 2     public boolean canWin(String s) {
 3         if (s==null || s.length()<=1) return false;
 4         for (int i=0; i<s.length()-1; i++) {
 5             if (s.charAt(i)==‘+‘ && s.charAt(i+1)==‘+‘ && !canWin(s.substring(0,i) + "--" + s.substring(i+2))) 
 6                 return true;
 7         }
 8         return false;
 9     }
10 }