Implement Stack and Queue using Deque

Difficulty Level : Easy
Last Updated : 14 Jan, 2021

Deque also known as double eneded queue, as name suggests is a special kind of queue in which insertions and deletions can be done at the last as well as at the beginning.

A link-list representation of deque is such that each node points to the next node as well as the previous node. So that insertion and deletions take constant time at both the beginning and the last.

Now, deque can be used to implement a stack and queue. One simply needs to understand how deque can made to work as a stack or a queue.
The functions of deque to tweak them to work as stack and queue are list below.

Examples: Stack

Input : Stack : 1 2 3
        Push(4)
Output : Stack : 1 2 3 4

Input : Stack : 1 2 3
        Pop()
Output : Stack : 1 2

Examples: Queue

Input: Queue : 1 2 3
       Enqueue(4)
Output: Queue : 1 2 3 4

Input: Queue : 1 2 3
       Dequeue()
Output: Queue : 2 3

C++

// C++ Program to implement stack and queue using Deque
#include <bits/stdc++.h>
using namespace std;
 
// structure for a node of deque
struct DQueNode {
    int value;
    DQueNode* next;
    DQueNode* prev;
};
 
// Implementation of deque class
class deque {
private:
 
    // pointers to head and tail of deque
    DQueNode* head;
    DQueNode* tail;
 
public:
    // constructor
    deque()
    {
        head = tail = NULL;
    }
 
    // if list is empty
    bool isEmpty()
    {
        if (head == NULL)
            return true;
        return false;
    }
 
    // count the number of nodes in list
    int size()
    {
        // if list is not empty
        if (!isEmpty()) {
            DQueNode* temp = head;
            int len = 0;
            while (temp != NULL) {
                len++;
                temp = temp->next;
            }
            return len;
        }
        return 0;
    }
 
    // insert at the first position
    void insert_first(int element)
    {
        // allocating node of DQueNode type
        DQueNode* temp = new DQueNode[sizeof(DQueNode)];
        temp->value = element;
 
        // if the element is first element
        if (head == NULL) {
            head = tail = temp;
            temp->next = temp->prev = NULL;
        }
        else {
            head->prev = temp;
            temp->next = head;
            temp->prev = NULL;
            head = temp;
        }
    }
 
    // insert at last position of deque
    void insert_last(int element)
    {
        // allocating node of DQueNode type
        DQueNode* temp = new DQueNode[sizeof(DQueNode)];
        temp->value = element;
 
        // if element is the first element
        if (head == NULL) {
            head = tail = temp;
            temp->next = temp->prev = NULL;
        }
        else {
            tail->next = temp;
            temp->next = NULL;
            temp->prev = tail;
            tail = temp;
        }
    }
 
    // remove element at the first position
    void remove_first()
    {
        // if list is not empty
        if (!isEmpty()) {
            DQueNode* temp = head;
            head = head->next;
            head->prev = NULL;
            free(temp);
            return;
        }
        cout << "List is Empty" << endl;
    }
 
    // remove element at the last position
    void remove_last()
    {
        // if list is not empty
        if (!isEmpty()) {
            DQueNode* temp = tail;
            tail = tail->prev;
            tail->next = NULL;
            free(temp);
            return;
        }
        cout << "List is Empty" << endl;
    }
 
    // displays the elements in deque
    void display()
    {
        // if list is not empty
        if (!isEmpty()) {
            DQueNode* temp = head;
            while (temp != NULL) {
                cout << temp->value << " ";
                temp = temp->next;
            }
            cout << endl;
            return;
        }
        cout << "List is Empty" << endl;
    }
};
 
// Class to implement stack using Deque
class Stack : public deque {
public:
    // push to push element at top of stack
    // using insert at last function of deque
    void push(int element)
    {
        insert_last(element);
    }
 
    // pop to remove element at top of stack
    // using remove at last function of deque
    void pop()
    {
        remove_last();
    }
};
 
// class to implement queue using deque
class Queue : public deque {
public:
    // enqueue to insert element at last
    // using insert at last function of deque
    void enqueue(int element)
    {
        insert_last(element);
    }
 
    // dequeue to remove element from first
    // using remove at first function of deque
    void dequeue()
    {
        remove_first();
    }
};
 
// Driver Code
int main()
{
    // object of Stack
    Stack stk;
 
    // push 7 and 8 at top of stack
    stk.push(7);
    stk.push(8);
    cout << "Stack: ";
    stk.display();
 
    // pop an element
    stk.pop();
    cout << "Stack: ";
    stk.display();
 
    // object of Queue
    Queue que;
 
    // insert 12 and 13 in queue
    que.enqueue(12);
    que.enqueue(13);
    cout << "Queue: ";
    que.display();
 
    // delete an element from queue
    que.dequeue();
    cout << "Queue: ";
    que.display();
 
    cout << "Size of Stack is " << stk.size() << endl;
    cout << "Size of Queue is " << que.size() << endl;
    return 0;
}

Java

// Java program to implement stack and 
// queue using Deque
class GFG{
 
// Class for a node of deque
static class DQueNode 
{
    int value;
    DQueNode next;
    DQueNode prev;
}
 
// Implementation of deque class
static class deque
{
     
    // Pointers to head and tail of deque
    private DQueNode head;
    private DQueNode tail;
 
    // Constructor
    public deque()
    {
        head = tail = null;
    }
     
    // If list is empty
    boolean isEmpty()
    {
        if (head == null)
            return true;
             
        return false;
    }
 
    // count the number of nodes in list
    int size()
    {
         
        // If list is not empty
        if (!isEmpty())
        {
            DQueNode temp = head;
            int len = 0;
             
            while (temp != null) 
            {
                len++;
                temp = temp.next;
            }
            return len;
        }
        return 0;
    }
 
    // Insert at the first position
    void insert_first(int element)
    {
         
        // Allocating node of DQueNode type
        DQueNode temp = new DQueNode();
        temp.value = element;
 
        // If the element is first element
        if (head == null) 
        {
            head = tail = temp;
            temp.next = temp.prev = null;
        }
        else
        {
            head.prev = temp;
            temp.next = head;
            temp.prev = null;
            head = temp;
        }
    }
 
    // Insert at last position of deque
    void insert_last(int element)
    {
         
        // Allocating node of DQueNode type
        DQueNode temp = new DQueNode();
        temp.value = element;
 
        // If element is the first element
        if (head == null) 
        {
            head = tail = temp;
            temp.next = temp.prev = null;
        }
        else
        {
            tail.next = temp;
            temp.next = null;
            temp.prev = tail;
            tail = temp;
        }
    }
 
    // Remove element at the first position
    void remove_first()
    {
         
        // If list is not empty
        if (!isEmpty())
        {
            DQueNode temp = head;
            head = head.next;
            head.prev = null;
 
            return;
        }
        System.out.print("List is Empty");
    }
 
    // Remove element at the last position
    void remove_last()
    {
         
        // If list is not empty
        if (!isEmpty()) 
        {
            DQueNode temp = tail;
            tail = tail.prev;
            tail.next = null;
 
            return;
        }
        System.out.print("List is Empty");
    }
 
    // Displays the elements in deque
    void display()
    {
         
        // If list is not empty
        if (!isEmpty()) 
        {
            DQueNode temp = head;
             
            while (temp != null) 
            {
                System.out.print(temp.value + " ");
                temp = temp.next;
            }
 
            return;
        }
        System.out.print("List is Empty");
    }
}
 
// Class to implement stack using Deque
static class Stack 
{
    deque d = new deque();
 
    // push to push element at top of stack
    // using insert at last function of deque
    public void push(int element)
    {
        d.insert_last(element);
    }
 
    // Returns size
    public int size()
    {
        return d.size();
    }
     
    // pop to remove element at top of stack
    // using remove at last function of deque
    public void pop()
    {
        d.remove_last();
    }
 
    // Display
    public void dispaly() 
    {
        d.display();
    }
}
 
// Class to implement queue using deque
static class Queue
{
    deque d = new deque();
     
    // enqueue to insert element at last
    // using insert at last function of deque
    public void enqueue(int element)
    {
        d.insert_last(element);
    }
 
    // dequeue to remove element from first
    // using remove at first function of deque
    public void dequeue() 
    {
        d.remove_first();
    }
 
    // display
    public void display() 
    { 
        d.display();
    }
     
    // size
    public int size() 
    {
        return d.size();
    }
}
 
// Driver Code
public static void main(String[] args)
{
     
    // Object of Stack
    Stack stk = new Stack();
 
    // push 7 and 8 at top of stack
    stk.push(7);
    stk.push(8);
    System.out.print("Stack: ");
    stk.dispaly();
 
    // For new line
    System.out.println();
     
    // pop an element
    stk.pop();
    System.out.print("Stack: ");
    stk.dispaly();
 
    // For new line
    System.out.println();
 
    // Object of Queue
    Queue que = new Queue();
 
    // Insert 12 and 13 in queue
    que.enqueue(12);
    que.enqueue(13);
    System.out.print("Queue: ");
    que.display();
 
    // New line
    System.out.println();
     
    // Delete an element from queue
    que.dequeue();
    System.out.print("Queue: ");
    que.display();
 
    // New line
    System.out.println();
    System.out.println("Size of stack is " + 
                       stk.size());
    System.out.println("Size of queue is " + 
                       que.size());
}
}
 
// This code is contributed by sujitmeshram

C#

// C# program to implement stack and
// queue using Deque
using System;
class GFG
{
 
    // Class for a node of deque
    public
 
        class DQueNode {
        public
 
            int value;
        public
            DQueNode next;
        public
            DQueNode prev;
    }
 
    // Implementation of deque class
    public class deque {
 
        // Pointers to head and tail of deque
        private DQueNode head;
        private DQueNode tail;
 
        // Constructor
        public deque() { head = tail = null; }
 
        // If list is empty
        public
 
            bool
            isEmpty()
        {
            if (head == null)
                return true;
 
            return false;
        }
 
        // count the number of nodes in list
        public
 
            int
            size()
        {
 
            // If list is not empty
            if (!isEmpty()) {
                DQueNode temp = head;
                int len = 0;
 
                while (temp != null) {
                    len++;
                    temp = temp.next;
                }
                return len;
            }
            return 0;
        }
 
        // Insert at the first position
        public
 
            void
            insert_first(int element)
        {
 
            // Allocating node of DQueNode type
            DQueNode temp = new DQueNode();
            temp.value = element;
 
            // If the element is first element
            if (head == null) {
                head = tail = temp;
                temp.next = temp.prev = null;
            }
            else {
                head.prev = temp;
                temp.next = head;
                temp.prev = null;
                head = temp;
            }
        }
 
        // Insert at last position of deque
        public
 
            void
            insert_last(int element)
        {
 
            // Allocating node of DQueNode type
            DQueNode temp = new DQueNode();
            temp.value = element;
 
            // If element is the first element
            if (head == null) {
                head = tail = temp;
                temp.next = temp.prev = null;
            }
            else {
                tail.next = temp;
                temp.next = null;
                temp.prev = tail;
                tail = temp;
            }
        }
 
        // Remove element at the first position
        public
 
            void
            remove_first()
        {
 
            // If list is not empty
            if (!isEmpty()) {
                head = head.next;
                head.prev = null;
 
                return;
            }
            Console.Write("List is Empty");
        }
 
        // Remove element at the last position
        public
 
            void
            remove_last()
        {
 
            // If list is not empty
            if (!isEmpty()) {
                tail = tail.prev;
                tail.next = null;
 
                return;
            }
            Console.Write("List is Empty");
        }
 
        // Displays the elements in deque
        public
 
            void
            display()
        {
 
            // If list is not empty
            if (!isEmpty()) {
                DQueNode temp = head;
 
                while (temp != null) {
                    Console.Write(temp.value + " ");
                    temp = temp.next;
                }
 
                return;
            }
            Console.Write("List is Empty");
        }
    }
 
    // Class to implement stack using Deque
    public class Stack {
        deque d = new deque();
 
        // push to push element at top of stack
        // using insert at last function of deque
        public void push(int element)
        {
            d.insert_last(element);
        }
 
        // Returns size
        public int size() { return d.size(); }
 
        // pop to remove element at top of stack
        // using remove at last function of deque
        public void pop() { d.remove_last(); }
 
        // Display
        public void dispaly() { d.display(); }
    }
 
    // Class to implement queue using deque
    class Queue {
        deque d = new deque();
 
        // enqueue to insert element at last
        // using insert at last function of deque
        public void enqueue(int element)
        {
            d.insert_last(element);
        }
 
        // dequeue to remove element from first
        // using remove at first function of deque
        public void dequeue() { d.remove_first(); }
 
        // display
        public void display() { d.display(); }
 
        // size
        public int size() { return d.size(); }
    }
 
    // Driver Code
    public static void Main(String[] args)
    {
 
        // Object of Stack
        Stack stk = new Stack();
 
        // push 7 and 8 at top of stack
        stk.push(7);
        stk.push(8);
        Console.Write("Stack: ");
        stk.dispaly();
 
        // For new line
        Console.WriteLine();
 
        // pop an element
        stk.pop();
        Console.Write("Stack: ");
        stk.dispaly();
 
        // For new line
        Console.WriteLine();
 
        // Object of Queue
        Queue que = new Queue();
 
        // Insert 12 and 13 in queue
        que.enqueue(12);
        que.enqueue(13);
        Console.Write("Queue: ");
        que.display();
 
        // New line
        Console.WriteLine();
 
        // Delete an element from queue
        que.dequeue();
        Console.Write("Queue: ");
        que.display();
 
        // New line
        Console.WriteLine();
        Console.WriteLine("Size of stack is " + stk.size());
        Console.WriteLine("Size of queue is " + que.size());
    }
}
 
// This code contributed by gauravrajput1

Output:

Stack: 7 8
Stack: 7
Queue: 12 13
Queue: 13
Size of Stack is 1
Size of Queue is 1

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