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 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 [].

 1 public class Solution {
 2     public List<String> generatePossibleNextMoves(String s) {
 3         List<String> res = new ArrayList<String>();
 4         if (s==null || s.length()<=1) return res;
 5         for (int i=0; i<s.length()-1; i++) {
 6             if (s.charAt(i)==‘+‘ &&s.charAt(i+1)==‘+‘) 
 7                 res.add(process(s, i));
 8         }
 9         return res;
10     }
11     
12     public String process(String s, int i) {
13         StringBuffer result = new StringBuffer();
14         result.append(s.substring(0, i));
15         result.append("--");
16         result.append(s.substring(i+2));
17         return result.toString();
18     }
19 }