Intersection of Two Arrays
Given two arrays num1 and num2 , return an array of their intersection. Each element in the result must be unique and you may return the result in any order.
Example 1 - Input: nums1 = [1,2,2,1], nums2 = [2,2] Output: 
Example 2 - Input: nums1 = [4,9,5], nums2 = [9,4,9,8,4] Output: [9,4] Explanation: [4,9] is also accepted.
- 1 <= nums1.length, nums2.length <= 1000
- 0 <= nums1[i], nums2[i] <= 1000
use dict/hashmap to record all nums appeared in the first list, and then check if there are nums in the second list have appeared in the map.