0454. 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


// 1) Hash Map
/**
 * @param {number[]} A
 * @param {number[]} B
 * @param {number[]} C
 * @param {number[]} D
 * @return {number}
 */
var fourSumCount = function(A, B, C, D) {
  const map = {}
  let count = 0
  for (const a of A) {
    for (const b of B) {
      map[a + b] = (map[a + b] || 0) + 1
    }
  }
  for (const c of C) {
    for (const d of D) {
      count += map[- c - d] || 0
    }
  }
  return count
}
// Runtime: 336 ms, faster than 45.02% of JavaScript online submissions for 4Sum II.
// Memory Usage: 78.7 MB, less than 50.00% of JavaScript online submissions for 4Sum II.
// console.log(fourSumCount([0, 1, -1], [-1, 1, 0], [0, 0, 1], [-1, 1, 1]));