Can you explain the concept of multiset and how it differs from a regular set?

A multiset (multi-set) is a collection of elements where repetitions are allowed. Unlike a regular set, where each element can appear only once, in a multiset the same element can be stored multiple times.

For example, in C++ STL, there is a container called std::multiset, which stores elements in sorted order and allows duplicates:

#include <iostream>
#include <set>

int main() {
    std::multiset<int> ms = {1, 2, 2, 3, 3, 3};
    for (int x : ms) {
        std::cout << x << " ";
    }
    // Output: 1 2 2 3 3 3
}

Thus, the main difference between a multiset and a set is the ability to store multiple identical elements.