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.

Example:

Input: s = "++++"
Output: 
[
  "--++",
  "+--+",
  "++--"
]

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

vector<string> generatePossibleNextMoves(string s) { // time: O(n); space: O(n)
    vector<string> res;
    if (s.empty()) return res;
    for (int i = 0; i < s.length() - 1; ++i) {
        if (s[i] == '+' && s[i + 1] == '+') {
            s[i] = s[i + 1] = '-';
            res.push_back(s);
            s[i] = s[i + 1] = '+';
        }
    }
    return res;
}