294. Flip Game II

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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.

For example, given s = "++++", return true. The starting player can guarantee a win by flipping the middle "++" to become "+--+".

Follow up:
Derive your algorithm's runtime complexity.

使用递归，核心思想是如果第一名玩家把两个‘+’换成‘-’之后，第二名玩家没有可以更换的判定是false，那么第一名玩家肯定是true。代码如下：

public class Solution {    public boolean canWin(String s) {        return helper(s.toCharArray());    }        private boolean helper(char[] chs) {        for (int i = 0; i < chs.length - 1; i ++) {            if (chs[i] == '+' && chs[i + 1] == '+') {                chs[i] = '-'; chs[i + 1] = '-';                boolean win = !helper(chs);                chs[i] = '+'; chs[i + 1] = '+';                if (win) {                    return true;                }            }        }        return false;    }}

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