In C++, finding the union of two multimaps consists of combining the elements from both multimap collections while considering the duplicates as a multimap allows multiple values to have the same key. In this article, we will learn to find the union of two multimaps in C++.

Example:

Input:
multi1 = {1, "Java"}, {2, "Python"}, {3, "C++"}, {4, "JavaScript"}
mutli2 = {2, "Python"}, {4, "JavaScript"}, {5, "TypeScript"} 

Output: 
Union MultiMap:
1: Java
2: Python
2: Python
3: C++
4: JavaScript
4: JavaScript
5: TypeScript

Finding Union of Two Multimaps in C++

To find the union of two multimaps in C++, we can insert all the elements of the two multimaps in the new multimap container using std::multimap::insert() function that inserts elements from a range to the multimap container.

C++ Program to Find the Union of Two Multimaps

The below program demonstrates how we can use the std::insert() function to find the union of two multimaps in C++ STL.

// C++ Program to illustrate how to find the union of two multimaps
#include <iostream>
#include <map>
using namespace std;

int main()
{
    // Defining multimap1
    multimap<int, string> multimap1 = { { 1, "Java" },
                                     { 2, "Python" },
                                     { 3, "C++" },
                                     { 4, "JavaScript" } };

    // Defining multimap2
    multimap<int, string> multimap2 = { { 2, "Python" },
                                     { 4, "JavaScript" },
                                     { 5, "TypeScript" } };

    // Finding union between multimap1 and multimap2
    multimap<int, string> unionMap = multimap1;

    unionMap.insert(multimap2.begin(), multimap2.end());

    // Printing union
    cout << "Union MultiMap:" << endl;
    for (const auto& pair : unionMap) {
        cout << pair.first << ": " << pair.second << endl;
    }

    return 0;
}

Union MultiMap:
1: Java
2: Python
2: Python
3: C++
4: JavaScript
4: JavaScript
5: TypeScript

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

Note: We can also use the std::merge function from the <algorithm> library to find the union of two multimaps in C++.