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 twoconsecutive "++" 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 [].

Understand the problem:
The problem only asks for flipping "++" into "--", NOT -- to ++. So the solution is to move all consecutive "++" into "--".


Code (Java):

public class Solution {
    public List<String> generatePossibleNextMoves(String s) {
        List<String> result = new ArrayList<>();
        if (s == null || s.length() < 2) {
            return result;
        }
        
        for (int i = 0; i < s.length() - 1; i++) {
            if (s.charAt(i) == '+' && s.charAt(i + 1) == '+') {
                String s1 = s.substring(0, i);
                String s2 = "--";
                String s3 = s.substring(i + 2);
                String temp = s1 + s2 + s3;
                result.add(temp);
            }
        }
        
        return result;
    }
}