How to Find Union of Two Multisets in C++?

In C++, multisets are a type of associative container similar to the set, with the exception that multiple elements can have the same values. Union is In this article, we will see how to find the union of two multisets in C++ STL.

Example

Input: 
first_multiset = {100,300,400,500,200,500,200}, 
second_multiset = {700,600,1000,500,500,500,400}

Output:
union = {100, 200, 200, 300, 400, 500, 500, 500, 600, 700, 1000}

Finding Union of Two Multisets in C++

To calculate the union of two multisets in C++, the std::set_union() function is provided by STL which is used to find the union of two sorted ranges. If the elements from the second multiset that have an equivalent element in the first multiset are not copied to the resulting container (Output container).

Syntax of std::set_union()

set_union (first1, last1, first2, last2, result);

where,

• first1: Iterator to the beginning of the first range.
• last1: Iterator to the last of the first range.
• first2: Iterator to the beginning of the second range.
• last1: Iterator to the last of the second range.
• result: Iterator to the beginning of the resulant data container.
  

C++ Program to Find Union of Two Multisets

C++

// C++ program to find union of two multisets.
#include <bits/stdc++.h>
using namespace std;

int main()
{
    // Create first_multiset with integer type
    // and insert 7 elements into it
    multiset<int> first_multiset;
    first_multiset.insert(100);
    first_multiset.insert(300);
    first_multiset.insert(400);
    first_multiset.insert(500);
    first_multiset.insert(200);
    first_multiset.insert(500);
    first_multiset.insert(200);

    // Create second_multiset with integer type
    // and insert 7 elements into it
    multiset<int> second_multiset;
    second_multiset.insert(700);
    second_multiset.insert(600);
    second_multiset.insert(1000);
    second_multiset.insert(500);
    second_multiset.insert(500);
    second_multiset.insert(500);
    second_multiset.insert(400);

    // Displaying the first_multiset
    cout << "First multiset: \n";
    multiset<int>::iterator itr1;
    for (itr1 = first_multiset.begin();
         itr1 != first_multiset.end(); ++itr1) {
        cout << *itr1 << ", ";
    }
    cout << endl;
    cout << endl;

    // Displaying the second_multiset
    cout << "Second multiset: \n";
    multiset<int>::iterator itr2;
    for (itr2 = second_multiset.begin();
         itr2 != second_multiset.end(); ++itr2) {
        cout << *itr2 << ", ";
    }
    cout << endl;
    cout << endl;

    // Create vector to store the union of two multisets
    // with size of both the multisets
    vector<int> v(first_multiset.size()
                  + second_multiset.size());

    vector<int>::iterator it, st;

    // Union of two multisets
    it = set_union(first_multiset.begin(),
                   first_multiset.end(),
                   second_multiset.begin(),
                   second_multiset.end(), v.begin());

    // Display vector
    cout << "Union of the above multisets: \n";
    for (st = v.begin(); st != it; ++st)
        cout << *st << ", ";

    return 0;
}

Output

First multiset: 
100, 200, 200, 300, 400, 500, 500, 

Second multiset: 
400, 500, 500, 500, 600, 700, 1000, 

Union of the above multisets: 
100, 200, 200, 300, 400, 500, 500, 500, 600, 700, 1000, 

Time complexity:O(N+M), where N and M is the size of the two multisets.
Auxiliary Space: O(N+M)