Flip Game

 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 class Solution {
    public List<String> generatePossibleNextMoves(String s) {
       List<String> res = new ArrayList<>();
       if (s == null || s.length() == 0) return res;
       char cs[] = s.toCharArray();
       for (int i = 1; i < s.length(); i++) {
           if (cs[i] == cs[i - 1] && cs[i] == '+') {
               cs[i] = '-';
               cs[i - 1] = '-';
               res.add(new String(cs));
               cs[i] = '+';
               cs[i - 1] = '+';
           }
       }
       return res;
    }
}