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350. Intersection of Two Arrays II — LeetCode

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

Example:
Given nums1 = [1, 2, 2, 1], nums2 = [2, 2], return [2, 2].

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?

Solution1: Hashtable
Count how many times each digit appears in nums1, store in hash table, where key is the number in array, value is the times.
Then iterate through nums2 array, if the number in nums2 is key of the hashtable, then check if the value larger then 1, if yes, store that number to result array.