350. Intersection of Two Arrays II

Problem

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?

Problem Summary

This problem is an enhanced version of Problem 349. It requires outputting the intersection elements of two arrays; if an element appears multiple times, it should be output multiple times.

Solution Approach

This problem still follows the approach of Problem 349. Put all the numbers in the first array into a map, where the key is a number from the array and the value is the number of times that number appears. When scanning the second array, each time an existing number is found, decrement the corresponding value in the map by one. If the value is 0, it means this number no longer exists.

Code

package leetcode

func intersect(nums1 []int, nums2 []int) []int {
	m := map[int]int{}
	var res []int
	for _, n := range nums1 {
		m[n]++
	}
	for _, n := range nums2 {
		if m[n] > 0 {
			res = append(res, n)
			m[n]--
		}
	}
	return res
}