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:
1. [
2. "--++",
3. "+--+",
4. "++--"
5. ]

If there is no valid move, return an empty list [].
Solution:
1. public class Solution {
2. public List generatePossibleNextMoves(String s) {
3. List res = new ArrayList<>();
4. 5. // sanity check
6. if (s == null || s.length() <= 1)
7. return res;
8. 9. for (int i = 0; i < s.length() - 1; i++)
10. if (s.startsWith("++", i))
11. res.add(s.substring(0, i) + "--" + s.substring(i + 2));
12. 13. return res;
14. }
15. }