You are playing the following Flip Game with your friend: Given a string that contains only these two characters:
-, you and your friend take turns to flip two consecutive
"--". 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