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 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.

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.

Solution: backtracking, recursion

public class Solution {
    public boolean canWin(String s) {
        if (s == null || s.length() < 2) {
            return false;
        }
        
        for (int i = 0; i < s.length() - 1; i++) {
            if (s.substring(i, i + 2).equals("++")) {
                if (canWin(s.substring(0, i) + "--" + s.substring(i + 2)) == false) {
                    return true;
                }
            }
        }
        
        return false;
    }
}