Similar Problems
Similar Problems not available
4sum Ii  Leetcode Solution
LeetCode: 4sum Ii Leetcode Solution
Difficulty: Medium
Topics: hashtable array
The problem statement for 4Sum II on LeetCode is as follows:
Given four lists A, B, C, D of integer values, compute how many tuples (i, j, k, l) there are such that A[i] + B[j] + C[k] + D[l] is zero.
To solve this problem, we can use a hash table to store all possible sums of two arrays A and B. We can then iterate over all possible pairs of arrays C and D, and check how many pairs of sums from A and B add up to the negative of the current sum from C and D.
Here’s how we can implement this solution:

Initialize a hash table to store the counts of sums of pairs from A and B.

Loop through the lists A and B, and add all possible sums to the hash table with their count.
from collections import defaultdict
def fourSumCount(A: List[int], B: List[int], C: List[int], D: List[int]) > int:
sums = defaultdict(int)
for a in A:
for b in B:
sums[a+b] += 1
 Loop through the lists C and D, and for each pair (c,d), check how many pairs of sums from A and B add up to (c+d).
count = 0
for c in C:
for d in D:
count += sums[(c+d)]
return count
Overall, the time complexity of this solution is O(n^2) because we are iterating through all possible pairs of arrays A, B, C, and D. However, the space complexity is also O(n^2) because we are storing all possible sums of pairs from A and B in a hash table.
4sum Ii Solution Code
1