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.
Input: s = "++++"
Explanation: The starting player can guarantee a win by flipping the middle "++" to become "+--+".
Derive your algorithm’s runtime complexity.