[Leetcode] 294. Flip Game II 解题报告

题目：

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.

思路：

这里需要注意一个转换：你自己可以保证赢的充要条件是你走完某一步之后，你的对手不可能赢。所以我们维护一个哈希表，里面记录所有不可能赢的字符串，一旦发现你走了某一步使得对手不可能赢，那么你就可以保证赢，否则你就不能保证你会赢。

代码：

class Solution {
public:
    bool canWin(string s) {
        if (s.length() <= 1) {
            return false;
        }
        if(hash.count(s)) {
            return false;
        }
        for(int i = 0; i < s.length() - 1; ++i) {
            if (s[i] == '+' && s[i + 1] == '+') {
                string str(s);
                str[i] = str[i + 1] = '-';
                if(!canWin(str)) {   // if the opposite cannot win, then you can gaurantee to win
                    hash.insert(str);
                    return true;
                }
            }
        }
        return false;
    }
private:
    unordered_set<string> hash;
};