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 [].