package com.company;

import java.util.HashMap;

import java.util.Map;

public class FourSumCount {

public int fourSumCount(int[] A, int[] B, int[] C, int[] D) {

Map<Integer, Integer> map = new HashMap<>();

// key: sum, value: frequency of sum.

int n = A.length;

int result = 0;

// Get all possible two-sums between C and D.

for (int i = 0; i < n; i++) {

for (int j = 0; j < n; j++) {

int sum = C[i] + D[j];

map.put(sum, map.getOrDefault(sum, 0) + 1);

}

}

// Count the number of two-sums

// between A and B that equals to

// opposite of any two-sum between C and D

for (int i = 0; i < n; i++) {

for (int j = 0; j < n; j++) {

int sum = A[i] + B[j];

if (map.containsKey(-sum))

result += map.get(-sum);

}

}

return result;

}

}