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?

######思路 将较短的数组缓存在hashMap中，花掉O(N)的辅助空间 然后遍历较长的数组，看是否在hashMap中，由于hashMap的O(1)查找时间，因此时间复杂度为O(M)