Problem description:

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

Example 1:

Input: nums1 = [1,2,2,1], nums2 = [2,2]
Output: [2]
Example 2:

Input: nums1 = [4,9,5], nums2 = [9,4,9,8,4]
Output: [9,4]
Note:

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

Solution:

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class Solution {
public:
    vector<int> intersection(vector<int>& nums1, vector<int>& nums2) {
        //chekc how many distinct element are the same
        unordered_set<int> check(nums1.begin(), nums1.end());
        vector<int> res;
        for(auto n: nums2){
            if(check.count(n)){
                res.push_back(n);
                check.erase(n);
            }
        }
        return res;
    }
};

time complexity: $O(n)$

reference:
https://goo.gl/ov6Mgb