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.

public class Solution {
    public List<String> generatePossibleNextMoves(String s) {
        List<String> result = new ArrayList<>();
        if (s == null || s.length() == 0) return result;

        for (int i = 1; i < s.length(); i++) {
            if (s.charAt(i - 1) == '+' && s.charAt(i) == '+') {
                result.add(s.substring(0, i - 1) + "--" + s.substring(i + 1));
            }
        }
        return result;
    }
}

class Solution(object):
    def generatePossibleNextMoves(self, s):
        """
        :type s: str
        :rtype: List[str]
        """

        if not s:
            return []

        ret = []
        for i in xrange(1, len(s)):
            if s[i] == "+" and s[i-1] == "+":
                ret.append(s[:i-1] + "--" + s[i+1:])
        return ret