[Leetcode / hash table] intersection of two arrays (hash table)

Problem Description:

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

Example 1:

输入: nums1 = [1,2,2,1], nums2 = [2,2]
输出: [2,2]

Example 2:

输入: nums1 = [4,9,5], nums2 = [9,4,9,8,4]
输出: [4,9]

Description:

• The output frequency of each element appears, should be consistent with the number of elements that appear in both arrays.
• We can not consider the order of output.

Advanced:

• If given array is already sorted it? How will you optimize your algorithm?
• If  nums1  size than  nums2  much smaller, which method is better?
• If  nums2  elements stored on disk, the disk memory is limited and you can not be loaded once all the elements into memory, how can you do?

The basic idea:

According to mathematical definition multi-set intersection.

We have established two hashmap, respectively, record the number of each element appears.

Then taking the smallest result set is added to the line.

AC Code:

class Solution {
public:
    vector<int> intersect(vector<int>& nums1, vector<int>& nums2) {
      map<int, int> hashmap1, hashmap2;
      for (auto num : nums1) {
        ++hashmap1[num];
      }
      for (auto num : nums2) {
        ++hashmap2[num];
      }
      for (auto it = hashmap1.begin(); it != hashmap1.end(); ++it) {
        it->second = (it->second < hashmap2[it->first])? it->second : hashmap2[it->first];
      }
      vector<int> res;
      for (auto it = hashmap1.begin(); it != hashmap1.end(); ++it) {
          for (int i = 0; i < it->second; ++i) {
            res.push_back(it->first);  
          }
      }
      return res;
    }
};