C++ Program to Implement Stack Using Two Queues

Stack

The stack which is implemented as LIFO, where insertion and deletion are done from the same end, top. The last element that entered is deleted first.

Stack operations are −

• push (int data) − Insertion at top
• int pop() − Deletion from top

Queue

The queue which is implemented as FIFO where insertions are done at one end (rear) and deletions are done from another end (front). The first element that entered is deleted first.

Queue operations are −

• EnQueue (int data) − Insertion at the rear end
• int DeQueue() − Deletion from front end

This is a C++ Program to Implement Stack Using Two Queues

Algorithms

Begin
   function enqueue1 to insert item a at qu1:
   Set, np1 = new qu1
   np1->d1 = a
   np1->n1 = NULL
   if (f1 == NULL)
      Then set
      r1 = np1
      r1->n1 = NULL
      f1 = r1
   else
      r1->n1 = np1
      r1 = np1
      r1->n1 = NULL
End

Begin
   function dequeue1 to delete item from qu1.
   if queue is null
      Print no elements present in queue.
   Else
      q1 = f1
      f1 = f1->n1
      a = q1->d1
      delete(q1)
   return a
End

Begin
   function enqueue2 to insert item a at qu2.
   np2 = new qu2;
   np2->d2 = a;
   np2->n2 = NULL;
   if queue is null
      Set r2 = np2
      r2->n2 = NULL
      f2 = r2
   Else
      Set r2->n2 = np2
      r2 = np2
      r2->n2 = NULL
End

Begin
   function dequeue2 to delete item from qu2:
   if queue is null
      Print no elements present in queue.
   Else
      q2 = f2
      f2 = f2->n2
      a = q2->d2
      delete(q2)
   return a
End

Example Code

#include<iostream>
using namespace std;

struct qu1// queue1 declaration {
   qu1 *n1;
   int d1;
}*f1 = NULL, *r1 = NULL, *q1 = NULL, *p1 = NULL, *np1 = NULL;

struct qu2// queue2 declaration {
   qu2 *n2;
   int d2;
}*f2 = NULL, *r2 = NULL, *q2 = NULL, *p2 = NULL, *np2 = NULL;

void enqueue1(int a) {
   np1 = new qu1;
   np1->d1 = a;
   np1->n1 = NULL;
   if (f1 == NULL) {
      r1 = np1;
      r1->n1 = NULL;
      f1 = r1;
   } else {
      r1->n1 = np1;
      r1 = np1;
      r1->n1 = NULL;
   }
}

int dequeue1() {
   int a;
   if (f1 == NULL) {
      cout<<"no elements present in queue\n";
   } else {
      q1 = f1;
      f1 = f1->n1;
      a = q1->d1;
      delete(q1);
      return a;
   }
}

void enqueue2(int a) {
   np2 = new qu2;
   np2->d2 = a;
   np2->n2 = NULL;
   if (f2 == NULL) {
      r2 = np2;
      r2->n2 = NULL;
      f2 = r2;
   } else {
      r2->n2 = np2;
      r2 = np2;
      r2->n2 = NULL;
   }
}

int dequeue2() {
   int a;
   if (f2 == NULL) {
      cout<<"no elements present in queue\n";
   } else {
      q2 = f2;
      f2 = f2->n2;
      a = q2->d2;
      delete(q2);
      return a;
   }
}

int main() {
   int n, a, i = 0;
   cout<<"Enter the number of elements to be entered into stack\n";
   cin>>n;
   while (i < n) {
      cout<<"enter the element to be entered\n";
      cin>>a;
      enqueue1(a);
      i++;
   }
   cout<<"\n\nElements popped\n\n";
   while (f1 != NULL || f2 != NULL)// if both queues are not null {
      if (f2 == NULL)// if queue 2 is null {
         while (f1->n1 != NULL) {
            enqueue2(dequeue1());
         }
         cout<<dequeue1()<<endl;
      } else if (f1 == NULL)//if queue 1 is null {
         while (f2->n2 != NULL) {
            enqueue1(dequeue2());
         }
         cout<<dequeue2()<<endl;
      }
   }
}

Output

Enter the number of elements to be entered into stack
5
enter the element to be entered
1
enter the element to be entered
2
enter the element to be entered
3
enter the element to be entered
4
enter the element to be entered
5

Elements popped
5
4
3
2
1