454 四数相加 II

https://leetcode.cn/leetbook/read/top-interview-questions-hard/xwqm6c/

cost: 12m21s

class Solution {
public:
    int fourSumCount(vector<int>& nums1, vector<int>& nums2, vector<int>& nums3,
                     vector<int>& nums4) {
        unordered_map<int, int> m;
        for (const auto num1 : nums1) {
            for (const auto num2 : nums2) {
                m[num1 + num2]++;
            }
        }
        unordered_map<int, int> m2;
        for (const auto num3 : nums3) {
            for (const auto num4 : nums4) {
                m2[num3 + num4]++;
            }
        }
        int res = 0;
        for (const auto [key, val] : m) {
            for (const auto [key2, val2] : m2) {
                if (key + key2 == 0) {
                    res += val * val2;
                }
            }
        }
        return res;
    }
};