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 determine if the starting player can guarantee a win.
Example:
Input:s = "++++"Output: true Explanation: The starting player can guarantee a win by flipping the middle"++"to become"+--+".
Follow up:
Derive your algorithm's runtime complexity.