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.

Solution

public class Solution {
    public boolean canWin(String s) {
        if (s == null || s.length() <= 1) return false;

        for (int i = 0; i < s.length() - 1; i++) {
            if (s.substring(i, i + 2).equals("++")) {
                StringBuffer sb = new StringBuffer(s);
                sb.replace(i, i + 2, "--");
                if (!canWin(sb.toString())) {
                    return true;
                }
            }
        }

        return false;
    }
}