LeetCode - Flip Game II

FlipGameII.java

// 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.

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 str = s.substring(0, i) + "--" + s.substring(i + 2);
            if(!canWin(str)) return true;
        }
    }
    
    return false;
}