#### clueless coding// TODO: be smarter

Cracking the Coding Interview Elements of Programming Interviews Programming Interviews Exposed # LeetCode 454. 4Sum II: C++ Solution

## Problem Statement

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 make problem a bit easier, all A, B, C, D have same length of N where 0 ≤ N ≤ 500. All integers are in the range of -228 to 228 - 1 and the result is guaranteed to be at most 231 - 1.

``````Function Signature (C++):
int fourSumCount(vector<int>& A, vector<int>& B, vector<int>& C, vector<int>& D)

Inputs:
A = [ 1, 2]
B = [-2,-1]
C = [-1, 2]
D = [ 0, 2]

Outputs:
2

We can create two different tuples:
(0, 0, 0, 1) -> A + B + C + D = 1 + (-2) + (-1) + 2 = 0
(1, 1, 0, 0) -> A + B + C + D = 2 + (-1) + (-1) + 0 = 0
``````

## TL;DR Code Solution

``````int fourSumCount(vector<int>& A, vector<int>& B, vector<int>& C, vector<int>& D) {
unordered_map<int, int> sums;
for (int i = 0; i < A.size(); i++) {
for (int j = 0; j < B.size(); j++) {
sums[A[i] + B[j]]++;
}
}

int count = 0;
for (int i = 0; i < C.size(); i++) {
for (int j = 0; j < D.size(); j++) {
if (sums.find(-(C[i] + D[j])) != sums.end()) {
count += sums[-(C[i] + D[j])];
}
}
}

return count;
}
``````