350. Intersection of Two Arrays II

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

Example 1:

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

Example 2:

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

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?

A:

class Solution {
public:
    vector<int> intersect(vector<int>& nums1, vector<int>& nums2) {
        map<int,int> m1;
        for(auto v : nums1)
        {
            if(m1.find(v) == m1.end())
            {
                m1.insert({v,1});
            }else{
                auto it = m1.find(v);
                it->second += 1;
            }
        }
        vector<int> res;
        for(auto v : nums2)
        {
            auto it = m1.find(v);
            if( it != m1.end())
            {
                res.push_back(v);
                it->second -= 1;
                if(it->second == 0)
                {
                    m1.erase(v);
                }
            }
        }
        return res;
    }
};

Mistakes: