Flip Game I && 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 compute all possible states of the string after one valid move.

For example, given s = "++++", after one move, it may become one of the following states:

[
  "--++",
  "+--+",
  "++--"
]

If there is no valid move, return an empty list [].

public List generatePossibleNextMoves(String s) {
        List rst = new ArrayList<>();
        for (int i = 0; i < s.length(); i++) {
            if (s.charAt(i) == '+' && s.charAt(i + 1) == '+') {
                rst.add(s.substring(0, i) + "--" + s.substring(i + 2));
            }
        }
        return rst;
    }

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.

The first one find all two consecutive "++"s and flip them to "--".

The second one find all two consecutive "++" and check if flipping them could lead a loss of the opponent.

public boolean canWin(String s) {
        for (int i = 0; i + 1 < s.length(); i++) {
            if (s.charAt(i) == '+' && s.charAt(i + 1) == '+') {
                if (!canWin(s.substring(0, i) + "-" + s.substring(i + 2)))
                    return true;
            }
        }
        return false;
    }