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:

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[
  "--++",
  "+--+",
  "++--"
]

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

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