> For the complete documentation index, see [llms.txt](https://jimmylin1991.gitbook.io/practice-of-algorithm-problems/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://jimmylin1991.gitbook.io/practice-of-algorithm-problems/string/293.-flip-game.md).

# 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 `[]`.

```cpp
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;
}
```