Intersection Of Two Arrays Ii

Problem

https://leetcode.com/problems/intersection-of-two-arrays-ii/

Given two integer arrays nums1 and nums2, return an array of their intersection. Each element in the result must appear as many times as it shows in both arrays and you may return the result in any order.

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]
Explanation: [9,4] is also accepted.

Constraints:

• 1 <= nums1.length, nums2.length <= 1000

• 0 <= nums1[i], nums2[i] <= 1000

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?

Pattern

Array, Hash Table, Two Pointers, Binary Search, Sorting

Approaches

Hash Map

Explanation

Count the number of times each number appears in each array in 2 dictionaries counter1 and counter2. For each key in counter1 and counter2, add key repeated

min(counter1[key], counter2[key])

times to the intersection list.

Code

from collections import Counter


def intersect(nums1: list[int], nums2: list[int]) -> list[int]:
    """Finds the elements in common between 2 arrays."""
    counter1 = Counter(nums1)
    counter2 = Counter(nums2)

    intersection = []
    for key, val in counter1.items():
        if key in counter2:
            repeat = min(val, counter2[key])
            intersection.extend(repeat * [key])

    return intersection

Test

>>> from intersection_of_two_arrays_ii__hash_map import intersect
>>> intersect([1,2,2,1], [2,2])
[2, 2]
>>> intersect([4,9,5], [9,4,9,8,4])
[4, 9]

Complexity

\(m\) is the length of nums1 and \(n\) is the length of nums2

Measure	Complexity	Notes
Time	\(O(m + n)\)	count each array and then iterate through the keys of the smaller dictionary
Auxiliary Space	\(O(m + n)\)	2 dictionaries store at most \(m\) and \(n\) key-value pairs respectively

intersection_of_two_arrays_ii__hash_map.intersect(nums1: list[int], nums2: list[int]) → list[int]

Finds the elements in common between 2 arrays.