Feb 3, 2022 - 7 min ' read

4Sum II

Problem Statement - link #

Given four integer arrays nums1, nums2, nums3, and nums4 all of length n, return the number of tuples (i, j, k, l) such that:

0 <= i, j, k, l < n
nums1[i] + nums2[j] + nums3[k] + nums4[l] == 0

Examples #

Example 1:

Input: nums1 = [1,2], nums2 = [-2,-1], nums3 = [-1,2], nums4 = [0,2]
Output: 2
Explanation:
The two tuples are:
1. (0, 0, 0, 1) -> nums1[0] + nums2[0] + nums3[0] + nums4[1] = 1 + (-2) + (-1) + 2 = 0
2. (1, 1, 0, 0) -> nums1[1] + nums2[1] + nums3[0] + nums4[0] = 2 + (-1) + (-1) + 0 = 0

Example 2:

Input: nums1 = [0], nums2 = [0], nums3 = [0], nums4 = [0]
Output: 1

Constraints #

n == nums1.length
n == nums2.length
n == nums3.length
n == nums4.length
1 <= n <= 200
-2^28 <= nums1[i], nums2[i], nums3[i], nums4[i] <= 2^28

Solutions #

class Solution {
public:
    int fourSumCount(vector<int>& nums1, vector<int>& nums2, vector<int>& nums3, vector<int>& nums4) {
      int count=0;
      unordered_map<int,int> mp;
      for (auto &it1: nums1)
            for (auto &it2:nums2)
                mp[it1+it2]++; 
      for(auto &it3: nums3)
            for(auto &it4:nums4)
               if(mp.count(0-it3-it4)) count+=mp[0-it3-it4];
      return count;   
    }
};