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?

参考之前Intersection of Two Arrays 这个题，对于follow up, sorted的显然2 pointer更好； size 1 小的话hashtable更好； nums2无法一次取出则表明无法sort，所以也用hashtable。

350. Intersection of Two Arrays II