You are playing a Flip Game with your friend. You are given a string currentState that contains only '+' 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. Return true if the starting player can guarantee a win, and false otherwise. Example 1: Input: currentState = "++++" Output: true Explanation: The starting player can guarantee a win by flipping the middle "++" to become "+--+". Example 2: Input: currentState = "+" Output: false Constraints: 1 <= currentState.length <= 60 currentState[i] is either '+' or '-'. Follow up: Derive your algorithm's runtime complexity.