1. Description

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.

2. Runtime Distribution

3. Submission Details

4. 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

5. Code

public int fourSumCount(int[] A, int[] B, int[] C, int[] D) {
    if (A == null || A.length == 0 || B == null || B.length == 0 || C == null || C.length == 0 || D == null
            || D.length == 0) {
        return 0;
    }

    Map<Integer, Integer> map = new HashMap<Integer, Integer>();
    for (int i = 0; i < A.length; i++) {
        for (int j = 0; j < B.length; j++) {
            int sum = A[i] + B[j];
            map.put(sum, map.getOrDefault(sum, 0) + 1);
        }
    }

    int result = 0;

    for (int i = 0; i < C.length; i++) {
        for (int j = 0; j < D.length; j++) {
            result += map.getOrDefault((~(C[i] + D[j])) + 1, 0);
        }
    }

    return result;
}

6.Test

import java.util.HashMap;
import java.util.Map;

public class LeetCode0454 {

    public int fourSumCount(int[] A, int[] B, int[] C, int[] D) {
        if (A == null || A.length == 0 || B == null || B.length == 0 || C == null || C.length == 0 || D == null
                || D.length == 0) {
            return 0;
        }

        Map<Integer, Integer> map = new HashMap<Integer, Integer>();
        for (int i = 0; i < A.length; i++) {
            for (int j = 0; j < B.length; j++) {
                int sum = A[i] + B[j];
                map.put(sum, map.getOrDefault(sum, 0) + 1);
            }
        }

        int result = 0;

        for (int i = 0; i < C.length; i++) {
            for (int j = 0; j < D.length; j++) {
                result += map.getOrDefault((~(C[i] + D[j])) + 1, 0);
            }
        }

        return result;
    }

    public static void main(String[] args) {
        int[] A = new int[] { 1, 2 };
        int[] B = new int[] { -2, -1 };
        int[] C = new int[] { -1, 2 };
        int[] D = new int[] { 0, 2 };
        LeetCode0454 leetcode = new LeetCode0454();
        System.out.println(leetcode.fourSumCount(A, B, C, D));
    }
}