LeetCode 350. Intersection of Two Arrays II

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?

class Solution {
public:
    vector<int> intersect(vector<int>& nums1, vector<int>& nums2) {
        vector<int> res;
        map<int,int> r1;
        for(auto num: nums1)
            r1[num]++;
        for(auto num: nums2)
            if(r1[num]>0){
                res.push_back(num);
                r1[num]--;
            }
        return res;
    }
};