293. 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:

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

please notice that the input string may be all '+', or all '-', or somewhere in between.

public class Solution {
    public List<String> generatePossibleNextMoves(String s) {
        List<String> res = new ArrayList<>();
        if(s.length() < 2) return res;
        for(int i=0; i<s.length()-1;i++){
            if(s.charAt(i) != '+' || s.charAt(i+1) != '+') continue;
            String t = s.substring(0, i) + "--" + s.substring(i+2);
            res.add(t);
        }

        return res;
    }
}