C++ Queue Exercises: Reverse the elements of a queue

Last update on December 20 2024 12:24:59 (UTC/GMT +8 hours)

Write a C++ program to reverse the elements of a queue.

Sample Solution:

C Code:

#include <iostream>
#include <stack> // Include the stack library for using stacks

using namespace std;

const int MAX_SIZE = 100;

class Queue {
private:
  int front;
  int rear;
  int arr[MAX_SIZE];

public:
  Queue() {
    front = -1; // Initialize front index to -1
    rear = -1; // Initialize rear index to -1
  }

  bool isFull() {
    return (rear == MAX_SIZE - 1); // Check if the queue is full
  }

  bool isEmpty() {
    return (front == -1 && rear == -1); // Check if the queue is empty
  }

  void enqueue(int x) {
    if (isFull()) {
      cout << "Error: Queue is full" << endl; // Display error message if queue is full
      return;
    }
    if (isEmpty()) {
      front = 0;
      rear = 0;
    } else {
      rear++;
    }
    arr[rear] = x; // Insert the element at the rear index
  }

  void dequeue() {
    if (isEmpty()) {
      cout << "Error: Queue is empty" << endl; // Display error message if queue is empty
      return;
    }
    if (front == rear) {
      front = -1;
      rear = -1;
    } else {
      front++;
    }
  }

  int peek() {
    if (isEmpty()) {
      cout << "Error: Queue is empty" << endl; // Display error message if queue is empty
      return -1;
    }
    return arr[front]; // Return the element at the front of the queue
  }

  void display() {
    if (isEmpty()) {
      cout << "Error: Queue is empty" << endl; // Display error message if queue is empty
      return;
    }
    cout << "Queue elements are: ";
    for (int i = front; i <= rear; i++) {
      cout << arr[i] << " "; // Display all elements in the queue
    }
    cout << endl;
  }

  // Function to reverse the elements of the queue
  void reverseQueue(Queue & q) {
    if (q.isEmpty()) {
      cout << "Error: Queue is empty" << endl; // Display error message if queue is empty
      return;
    }
    stack < int > s; // Create a stack to store elements temporarily
    while (!q.isEmpty()) {
      s.push(q.peek()); // Push each element of the queue to the stack
      q.dequeue(); // Remove the element from the queue
    }
    while (!s.empty()) {
      q.enqueue(s.top()); // Enqueue the elements from the stack to the queue
      s.pop(); // Pop the element from the stack
    }
  }
};

int main() {
  cout << "Initialize a Queue." << endl;
  Queue q;
  cout << "\nInsert some elements into the queue:" << endl;
  q.enqueue(1);
  q.enqueue(2);
  q.enqueue(3);
  q.enqueue(4);
  q.enqueue(5);
  q.display();
  q.reverseQueue(q); // Reverse the elements of the queue
  cout << "\nReverse Queue elements:\n";
  q.display();
  return 0;
}

Sample Output:

Initialize a Queue.

Insert some elements into the queue:
Queue elements are: 1 2 3 4 5

Reverse Queue elements:
Queue elements are: 5 4 3 2 1

Flowchart:

CPP Code Editor:

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Previous C++ Exercise: Implement a queue using an array.
Next C++ Exercise: Sort the elements of a queue.