454-4Sum-II.js

/**
 * 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
 */

/**
 * @param {number[]} A
 * @param {number[]} B
 * @param {number[]} C
 * @param {number[]} D
 * @return {number}
 */
var fourSumCount = function (A, B, C, D) {
    var cnt = 0;
    var map = {};
    var n = A.length;

    for (var i = 0; i < n; i++) {
        for (var j = 0; j < n; j++) {
            var sum = A[i] + B[j];
            if (map[sum] === undefined) map[sum] = 1;
            else map[sum]++;
        }
    }

    for (var i = 0; i < n; i++) {
        for (var j = 0; j < n; j++) {
            var sum = C[i] + D[j];
            if (map[-sum] !== undefined) cnt += map[-sum];
        }
    }
    return cnt;
};

console.log(fourSumCount([1, 2], [-2, -2], [-1, 2], [0, 2]));