LeetCode 350. Intersection of Two Arrays II



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]


  • 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?


use a hashmap to check intersections

Python Solution

class Solution:
    def intersect(self, nums1: List[int], nums2: List[int]) -> List[int]:
        counter = {}
        for num in nums1:
            if num in counter:
                counter[num] += 1
                counter[num] = 1
        result = []
        for num in nums2:
            if num in counter and counter[num] != 0:
                counter[num] -= 1
        return result
  • Time complexity: O(n+m), where n and m are the lengths of the arrays. We iterate through the first, and then through the second array; insert and lookup operations in the hash map take a constant time.
  • Space complexity: O(min(n,m)). We use a hash map to store numbers (and their counts) from the smaller array.

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