294. 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 runtime complexity. public boolean canWin(String s) { if (s == null || s.length() < 2) { return false; } for (int i = 0; i < s.length() - 1; i++) { if (s.startsWith("++", i)) { String t = s.substring(0, i) + "--" + s.substring(i + 2); if (!canWin(t)) { return true; } } } return false; }