Intersection of Two Arrays II

Description

Given two arrays, write a function to compute their intersection.

Example 1:

Input: nums1 = [1,2,2,1], nums2 = [2,2]
Output: [2,2]

Example 2:

Input: nums1 = [4,9,5], nums2 = [9,4,9,8,4]
Output: [4,9]

Note:

• Each element in the result should appear as many times as it shows in both arrays.
• The result can be in any order.

Follow up:

• What if the given array is already sorted? How would you optimize your algorithm?
• What if nums1's size is small compared to nums2's size? Which algorithm is better?
• What if elements of nums2 are stored on disk, and the memory is limited such that you cannot load all elements into the memory at once?

Solution(javascript)

/**
 * @param {number[]} nums1
 * @param {number[]} nums2
 * @return {number[]}
 */
const intersect = (nums1, nums2) => {
  const map = nums1.reduce(
    (acc, num1) => {
      if (acc[num1]) {
        acc[num1] += 1
      } else {
        acc[num1] = 1
      }
      return acc
    },
    {},
  )
  return nums2.filter((num2) => {
    if (map[num2]) {
      map[num2] -= 1
      return true
    }
    return false
  })
}