In C++, an unordered_multimap is an associative container that contains key-value pairs allowing multiple elements with the same key. In this article, we will learn how to create a stack of unordered_multimaps in C++.

Example:

Input:
myMultimap1 = { {1, “C++”}, {2, “Java”}, {1, “Python”} };
myMultimap2 = { {2, “JavaScript”}, {3, "R"  }};

Output:
myStack: [ { {1, “C++”}, {2, “Java”}, {1, “Python”} },
         { {2, “JavaScript”} } ]

Stack of Unordered_Multimaps in C++

To create a std::stack of std::unordered_multimaps in C++, we have to define the type of stack elements as an unordered_multimap in the template definition.

Syntax to Declare Stack of Unordered_Multimap

stack < unordered_multimap <keyType, valueType> > myStack;

Here,

• key_type is the type of key stored in the unordered_multimap
• value_type is the type of value stored in the unordered_multimap.
• myStack is the name of the stack of unordered_multimap.

C++ Program to Create Stack of Unordered_Multimap

The below program demonstrates how we can create and use a stack of unordered_multimap in C++ STL.

// C++ Program to illustrate how to create a stack of
// unordered_multimaps
#include <iostream>
#include <stack>
#include <unordered_map>

using namespace std;

int main()
{
    // Initialize an unordered_multimap with some entries
    unordered_multimap<int, string> myUnorderedMultimap1
        = { { 1, "C++" }, { 2, "Java" }, { 1, "Python" } };
    unordered_multimap<int, string> myUnorderedMultimap2
        = { { 2, "JavaScript" }, { 3, "R" } };
    ;

    // Create a stack of unordered_multimaps
    stack<unordered_multimap<int, string> > myStack;
    myStack.push(myUnorderedMultimap1);
    myStack.push(myUnorderedMultimap2);

    // Print unordered_multimap in the stack
    cout << "myStack:" << endl << "[";
    while (!myStack.empty()) {
        auto ele = myStack.top();
        cout << " { ";
        for (auto& pair : ele) {
            cout << "{" << pair.first << ", " << pair.second
                 << "}, ";
        }
        cout << " } ";
        myStack.pop();
    }
    cout << " ]";

    return 0;
}

myStack:
[ { {3, R}, {2, JavaScript},  }  { {1, Python}, {1, C++}, {2, Java},  }  ]

Time Complexity: O(N), here N is the total number of elements across all unordered_multimaps.
Auxiliary Space: O(N)