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:

[
  "--++",
  "+--+",
  "++--"
]
If there is no valid move, return an empty list [].

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