454.hpp

#ifndef LEETCODE_454_HPP
#define LEETCODE_454_HPP

#include <iostream>
#include <queue>
#include <algorithm>
#include <vector>
#include <unordered_map>
#include <unordered_set>
#include <set>
#include <numeric>
#include <stack>
#include <string>

using namespace std;

/*
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.

Example:

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

Output:
2

Explanation:
The two tuples are:
1. (0, 0, 0, 1) -> A[0] + B[0] + C[0] + D[1] = 1 + (-2) + (-1) + 2 = 0
2. (1, 1, 0, 0) -> A[1] + B[1] + C[0] + D[0] = 2 + (-1) + (-1) + 0 = 0

 */

class Solution {
public:
    int fourSumCount(vector<int> &A, vector<int> &B, vector<int> &C, vector<int> &D) {
        unordered_map<int, int> cache;
        for (auto a : A)
            for (auto b : B)
                cache[a + b]++;
        int cnt = 0;
        for (auto c : C)
            for (auto d : D)
                cnt += cache[0 - c - d];
        return cnt;
    }
};


#endif //LEETCODE_454_HPP