leetcode 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?

 

// class Solution {
// public:
//     vector<int> intersect(vector<int>& nums1, vector<int>& nums2) {
//         unordered_map<int, int> ctr;
//         vector<int> res;
//         for (auto i : nums1) ctr[i]++;
//         for (auto i : nums2) {
//             if (ctr.count(i) > 0 && ctr[i] > 0) {
//                 ctr[i]--;
//                 res.push_back(i);
//             }
//         }
//         return res;
//     }
// };

class Solution {
public:
    vector<int> intersect(vector<int>& nums1, vector<int>& nums2) {
        vector<int> res;
        
        sort(nums1.begin(), nums1.end());
        sort(nums2.begin(), nums2.end());
        
        int i1 = 0, i2 = 0;
        while(i1<nums1.size() && i2<nums2.size()){
            if(nums1[i1] == nums2[i2]){
                res.push_back(nums1[i1]);
                i1++;
                i2++;
            }
            else if(nums1[i1] < nums2[i2]){
                i1++;
            }
            else{
                i2++;
            }
        }
        
        return res;
    }
};