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 [].

Solution: run time complexity is O(n), space complexity is O(n).

public class Solution {
    public List generatePossibleNextMoves(String s) {
        List res = new ArrayList();
        if (s == null || s.length() <= 1) {
            return res;
        }
        
        int i = 0;
        while (i + 1 < s.length()) {
            if (s.charAt(i) == '+' && s.charAt(i + 1) == '+') {
                StringBuilder sb = new StringBuilder(s);
                sb.setCharAt(i, '-');
                sb.setCharAt(i + 1, '-');
                res.add(sb.toString());
            }
            i++;
        }
        
        return res;
    }
}

Or:

public class Solution {
    public List generatePossibleNextMoves(String s) {
        List res = new ArrayList();
        if (s == null || s.length() <= 1) {
            return res;
        }
        
        for (int i = 0; i < s.length() - 1; i++) {
            if (s.substring(i, i + 2).equals("++")) {
                res.add(s.substring(0, i) + "--" + s.substring(i + 2));
            }
        }
        
        return res;
    }
}