Given two arrays, write a function to compute their intersection.
Example:
Given nums1 = [1, 2, 2, 1], nums2 = [2, 2], return [2].

Note:

• Each element in the result must be unique.
• The result can be in any order.

https://discuss.leetcode.com/topic/45685/three-java-solutions

** 解題思路**
三種解法：

1. Use two hash sets , O(n) Time complexity
   先把nums1加入到set中，然后用nums2中元素在set中遍歷，如果set包含該元素，就加入到intersect set中。最后把intersect set 存入結果數組。

2. Sort both arrays, use two pointers O(nlogn) Time complexity

3. Binary search O(nlogn) Time complexity
   先sort nums2 ，然后用nums1元素在nums2中做binary search。如果能找到，就加入結果集。