LeetCode–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()){
            return res;
        }
        for (int i=1; i<s.length(); i++){
            if(s.charAt(i-1) == ‘+‘ && s.charAt(i) == ‘+‘){
                res.add(s.substring(0,i-1) + "--" + s.substring(i+1));
            }
        }
        return res;
    }
}