Implement Queue Using Two Stacks in C++

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 rear end
• int DeQueue() − Deletion from front end

This is a C++ Program to Implement Queue Using Two Stacks.

Description of functions

• function enQueue() to enqueue an item to queue:
  • Push m to s1.
• function deQueue() to dequeue items from queue.
  • If both the stacks empty then print queue is empty.
  • If s2 is empty then move elements from s1 to s2.
  • Pop the elements from s2 and return it.
• function push() to push item in the stack.
• function pop() to pop out an item from the stack.

Example Code

#include<stdlib.h>
#include<iostream>
using namespace std;

struct nod//node declaration {
   int d;
   struct nod *n;
};

void push(struct nod** top_ref, int n_d);//functions prototypes.
int pop(struct nod** top_ref);

struct queue {
   struct nod *s1;
   struct nod *s2;
};

void enQueue(struct queue *q, int m) {
   push(&q->s1, m);
}

int deQueue(struct queue *q) {
   int m;
   if (q->s1 == NULL && q->s2 == NULL) {
      cout << "Queue is empty";
      exit(0);
   }
   if (q->s2 == NULL) {
      while (q->s1 != NULL) {
         m = pop(&q->s1);
         push(&q->s2, m);
      }
   }
   m = pop(&q->s2);
   return m;
}

void push(struct nod** top_ref, int n_d) {
   struct nod* new_node = (struct nod*) malloc(sizeof(struct nod));

   if (new_node == NULL) {
      cout << "Stack underflow \n";
      exit(0);
   }
   //put items on stack
   new_node->d= n_d;
   new_node->n= (*top_ref);
   (*top_ref) = new_node;
}

int pop(struct nod** top_ref) {
   int res;
   struct nod *top;
   if (*top_ref == NULL)//if stack is null {
      cout << "Stack overflow \n";
      exit(0);
   } else { //pop elements from stack
      top = *top_ref;
      res = top->d;
      *top_ref = top->n;
      free(top);
      return res;
   }
}

int main() {
   struct queue *q = (struct queue*) malloc(sizeof(struct queue));
   q->s1 = NULL;
   q->s2 = NULL;
   cout << "Enqueuing..7";
   cout << endl;
   enQueue(q, 7);
   cout << "Enqueuing..6";
   cout << endl;
   enQueue(q, 6);
   cout << "Enqueuing..2";
   cout << endl;
   enQueue(q, 2);
   cout << "Enqueuing..3";
   cout << endl;
   enQueue(q, 3);

   cout << "Dequeuing...";
   cout << deQueue(q) << " ";
   cout << endl;
   cout << "Dequeuing...";
   cout << deQueue(q) << " ";
   cout << endl;
   cout << "Dequeuing...";
   cout << deQueue(q) << " ";
   cout << endl;
}

Output

Enqueuing..7
Enqueuing..6
Enqueuing..2
Enqueuing..3
Dequeuing...7
Dequeuing...6
Dequeuing...2