Leetcode 679. 24 点游戏

题目

Leetcode 679. 24 点游戏

代码（9.24 首刷部分看解析）

class Solution {
public:
    static constexpr double dif = 1e-6;
    bool judgePoint24(vector<int>& cards) {
        vector<double> v(cards.begin(), cards.end());
        return backtrack(v);
    }
    double operation(double n1, double n2, int k) {
        switch(k) {
            case 0 :
                return n1+n2;
            case 1 :
                return n1-n2;
            case 2 :
                return n1*n2;
            case 3 :
                return n2-n1;
            case 4 :
                return n1/n2;
            case 5 : 
                return n2/n1;
        }
        return -1;
    }
    bool backtrack(vector<double>& cards) {
        if(cards.size() == 1) {
            return abs(cards[0]-24) <= dif;
        }
        for(int i = 0; i < cards.size(); i++) {
            for(int j = i+1; j < cards.size(); j++) {
                vector<double> next;
                for(int k = 0; k < cards.size(); k++) {
                    if(k == i || k == j) continue;
                    next.emplace_back(cards[k]);
                }
                double n1 = cards[i], n2 = cards[j];
                for(int q = 0; q < 6; q++) {
                    if(q == 4 && !n2 || q == 5 && !n1) continue;
                    next.emplace_back(operation(n1, n2, q));
                    if(backtrack(next)) return true;
                    next.pop_back();
                }
            }
        }
        return false;
    }
};