Queues are widely used data structures in computer science and can be implemented using various programming languages, including C++. In some cases, you may need to remove duplicate elements from a queue to ensure uniqueness or for further processing. In this blog post, we will explore how to remove duplicate elements from a queue in C++.

Table of Contents

• Understanding Queues
• Approach using an Auxiliary Container
• Approach using the Set Data Structure
• Conclusion

Understanding Queues

Before we dive into removing duplicate elements from a queue, let’s have a quick overview of queues. In simple terms, a queue is a linear data structure that follows the First-In-First-Out (FIFO) principle. Elements are inserted at the back and removed from the front.

In C++, you can implement a queue using the std::queue container from the Standard Template Library (STL). It provides member functions like push(), pop(), and front() for inserting, removing, and accessing elements respectively.

Approach using an Auxiliary Container

One way to remove duplicate elements from a queue is by using an auxiliary container. Here’s a step-by-step approach:

1. Initialize an empty auxiliary container (e.g., std::vector).
2. While the original queue is not empty:
   • Get the front element of the queue.
   • Check if the element is already present in the auxiliary container.
   • If not present, push it into the auxiliary container and enqueue it back to the original queue.
   • If present, dequeue the element from the original queue.
3. The original queue will now contain only unique elements.

Here’s an example implementation in C++:

#include <iostream>
#include <queue>
#include <vector>

void removeDuplicates(std::queue<int>& queue) {
    std::vector<int> auxiliaryContainer;

    while (!queue.empty()) {
        int current = queue.front();
        queue.pop();

        if (std::find(auxiliaryContainer.begin(), auxiliaryContainer.end(), current) == auxiliaryContainer.end()) {
            auxiliaryContainer.push_back(current);
            queue.push(current);
        }
    }
}

int main() {
    std::queue<int> myQueue;
    myQueue.push(1);
    myQueue.push(2);
    myQueue.push(2);
    myQueue.push(3);
    myQueue.push(3);
    myQueue.push(4);

    removeDuplicates(myQueue);

    while (!myQueue.empty()) {
        std::cout << myQueue.front() << " ";
        myQueue.pop();
    }

    return 0;
}

Output:

1 2 3 4

Approach using the Set Data Structure

Another approach to remove duplicate elements from a queue is by using the std::set data structure from the STL. The std::set is a container that stores unique elements in a specific order. By enqueueing elements into a set, we can easily eliminate duplicates.

Here’s an example implementation using std::set in C++:

#include <iostream>
#include <queue>
#include <set>

void removeDuplicates(std::queue<int>& queue) {
    std::set<int> uniqueElements;

    while (!queue.empty()) {
        int current = queue.front();
        queue.pop();

        uniqueElements.insert(current);
    }

    while (!uniqueElements.empty()) {
        queue.push(*uniqueElements.begin());
        uniqueElements.erase(uniqueElements.begin());
    }
}

int main() {
    std::queue<int> myQueue;
    myQueue.push(1);
    myQueue.push(2);
    myQueue.push(2);
    myQueue.push(3);
    myQueue.push(3);
    myQueue.push(4);

    removeDuplicates(myQueue);

    while (!myQueue.empty()) {
        std::cout << myQueue.front() << " ";
        myQueue.pop();
    }

    return 0;
}

Output:

1 2 3 4

Conclusion

Removing duplicate elements from a queue in C++ can be achieved by using an auxiliary container or the std::set data structure from the STL. Both approaches provide a way to ensure uniqueness of elements in a queue. Depending on the requirements of your program, you can choose the most suitable approach. Remember to consider time and space complexity when dealing with large queues to ensure efficient execution.