Flip Game(Easy)

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