class Solution {
    public int fourSumCount(int[] A, int[] B, int[] C, int[] D) {
        // A map to map all possible sum of elements in C and D to their count
        Map<Integer, Integer> m = new HashMap<>();

        for (int i = 0; i < C.length; i++) {
            for (int j = 0; j < D.length; j++) {
                int sum = C[i] + D[j];

                // Update counter
                // One more pair that adds up to sum
                m.put(sum, m.getOrDefault(sum, 0) + 1);
            }
        }

        // Going through A and B, if find A[i] + B[j] equals to -sum,
        // meaning we found a pair of A,B and can pair with all counts
        // of C, D that adds up to sum
        int result = 0;

        for (int i = 0; i < A.length; i++) {
            for (int j = 0; j < B.length; j++) {
                int sum = A[i] + B[j];

                result += m.getOrDefault(-sum, 0);
            }
        }

        return result;
    }
}