454__medium__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) {
  let map = new Map();
  let res = 0;

  for (let i = 0; i < A.length; i++) {
    for (let j = 0; j < B.length; j++) {
      let sum = A[i] + B[j];
      map.set(sum, (map.get(sum) || 0) + 1);
    }
  }
  for (let i = 0; i < C.length; i++) {
    for (let j = 0; j < D.length; j++) {
      let target = -C[i] - D[j];
      res += map.get(target) || 0;
    }
  }
  return res;
};
/**
 *
 * O(n^2) time (n for length of A)
 * O(n^2) space
 *
 * 15m
 *
 */