350. Intersection of Two Arrays II

• Difficulty: Easy

• Topics: Hash Table, Two Pointers, Binary Search, Sort

• Similar Questions:

  • Intersection of Two Arrays

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Problem:

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

Example 1:

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

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?

Solutions:

class Solution {
public:
    vector<int> intersect(vector<int>& nums1, vector<int>& nums2) {
        if (nums1.size() > nums2.size()) {
            return intersect(nums2, nums1);
        }

        unordered_multiset<int> numSet(nums1.begin(), nums1.end());

        vector<int> ret;
        for (auto num : nums2) {
            auto it = numSet.find(num);
            if (it != numSet.end()) {
                ret.push_back(num);
                numSet.erase(it);
            }
        }

        return ret;
    }
};