Flip Game -- LeetCode

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 [].

 class Solution {
 public:
     vector<string> generatePossibleNextMoves(string s) {
         vector<string> res;
         for (int i = , n = s.size() - ; i < n; i++)
             if (s[i] == '+' && s[i + ] == '+') {
                 string temp = s;
                 temp[i] = temp[i + ] = '-';
                 res.push_back(temp);
             }
         return res;
     }
 };