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]

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?

My solution:

class Solution:
    def intersect(self, nums1, nums2):
        """
        :type nums1: List[int]
        :type nums2: List[int]
        :rtype: List[int]
        """
        if not nums1 or not nums2:
            return []

        dict1 = {}
        for i in nums1:
            if i in dict1:
                dict1[i] = dict1[i] + 1
            else:
                dict1[i] = 1

        res = []
        for i in nums2:
            if i in dict1:
                if dict1[i] > 0:
                    dict1[i] = dict1[i] - 1
                    res.append(i)

        return res