package leetcode_1To300;

import java.util.HashMap;

/**
 * 本代码来自 Cspiration，由 @Cspiration 提供
 * 题目来源：http://leetcode.com
 * - Cspiration 致力于在 CS 领域内帮助中国人找到工作，让更多海外国人受益
 * - 现有课程：Leetcode Java 版本视频讲解（1-900题）（上）（中）（下）三部
 * - 算法基础知识（上）（下）两部；题型技巧讲解（上）（下）两部
 * - 节省刷题时间，效率提高2-3倍，初学者轻松一天10题，入门者轻松一天20题
 * - 讲师：Edward Shi
 * - 官方网站：https://cspiration.com
 * - 版权所有，转发请注明出处
 */
public class _294_FlipGameII {

    /**
     * 294. Flip Game II
     * 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 "+--+".

     HashMap<String, Boolean>


     * time : O(2^N) 不确定
     * space : O(2^N)
     * @param s
     * @return
     */

    public boolean canWin(String s) {
        if (s == null || s.length() == 0) return false;
        HashMap<String, Boolean> map = new HashMap<>();
        return helper(s, map);
    }

    private boolean helper(String s, HashMap<String, Boolean> map) {
        if (map.containsKey(s)) return map.get(s);
        for (int i = 0; i < s.length() - 1; i++) {
            if (s.charAt(i) == '+' && s.charAt(i + 1) == '+') {
                String opponent = s.substring(0, i) + "--" + s.substring(i + 2);
                if (!helper(opponent, map)) {
                    map.put(s, true);
                    return true;
                }
            }
        }
        map.put(s, false);
        return false;
    }
}