294. Flip Game II

zshouyi

于 2017-04-28 07:51:27 发布

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分类专栏： LeetCode Backtracking 文章标签： LeetCode Backtracking Flip Game II

本文链接：https://blog.youkuaiyun.com/zshouyi/article/details/70882263

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本文介绍了一个名为FlipGame的游戏，该游戏由两名玩家轮流将字符串中的连续两个'+'字符翻转为'-'，首先无法进行有效翻转的玩家输掉比赛。文章提供了一种递归算法来判断先手玩家是否能确保获胜，并附带了Java实现代码。

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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;
    }
}