[leetcode] 294. Flip Game II 解题报告

题目链接：https://leetcode.com/problems/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 "+--+".

Follow up:
Derive your algorithm's runtime complexity.

思路：枚举每一种情况，然后递归到下一层，看对手可不可以赢，如果对手有赢得情况那么就无法保证一定能赢。时间复杂度是指数。

还有O(n)时间内能解决的，不过有点麻烦，这个解面试一般够用了。有兴趣的再去找找吧。

代码如下：

class Solution {
public:
    bool canWin(string s) {
        int len = s.size();
        vector<string> strs;
        for(int i = 0; i< len-1; i++)
            if(s[i] == '+' && s[i+1] == '+')
                strs.push_back(s.substr(0, i)+"--"+s.substr(i+2));
        for(auto str: strs)
            if(!canWin(str))
                return true;
        return false;
    }
};

参考：https://leetcode.com/discuss/73327/o-n-time-o-1-space-c-solution