Write the Implementation (.cpp File) of the Counter Class of the Previous Exercise.
Implement Stack using Queues
The problem is opposite of this post. We are given a Queue data structure that supports standard operations like enqueue() and dequeue(). We need to implement a Stack data structure using only instances of Queue and queue operations allowed on the instances.
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A stack can be implemented using two queues. Let stack to be implemented be 's' and queues used to implement be 'q1' and 'q2'. Stack 's' can be implemented in two ways:
Method 1 (By making push operation costly)
This method makes sure that newly entered element is always at the front of 'q1', so that pop operation just dequeues from 'q1'. 'q2' is used to put every new element at front of 'q1'.
- push(s, x) operation's step are described below:
- Enqueue x to q2
- One by one dequeue everything from q1 and enqueue to q2.
- Swap the names of q1 and q2
- pop(s) operation's function are described below:
- Dequeue an item from q1 and return it.
Below is the implementation of the above approach:
C++
#include <bits/stdc++.h>
using namespace std;
class Stack {
queue< int > q1, q2;
int curr_size;
public :
Stack()
{
curr_size = 0;
}
void push( int x)
{
curr_size++;
q2.push(x);
while (!q1.empty()) {
q2.push(q1.front());
q1.pop();
}
queue< int > q = q1;
q1 = q2;
q2 = q;
}
void pop()
{
if (q1.empty())
return ;
q1.pop();
curr_size--;
}
int top()
{
if (q1.empty())
return -1;
return q1.front();
}
int size()
{
return curr_size;
}
};
int main()
{
Stack s;
s.push(1);
s.push(2);
s.push(3);
cout << "current size: " << s.size()
<< endl;
cout << s.top() << endl;
s.pop();
cout << s.top() << endl;
s.pop();
cout << s.top() << endl;
cout << "current size: " << s.size()
<< endl;
return 0;
}
Java
import java.util.*;
class GfG {
static class Stack {
static Queue<Integer> q1 = new LinkedList<Integer>();
static Queue<Integer> q2 = new LinkedList<Integer>();
static int curr_size;
Stack()
{
curr_size = 0 ;
}
static void push( int x)
{
curr_size++;
q2.add(x);
while (!q1.isEmpty()) {
q2.add(q1.peek());
q1.remove();
}
Queue<Integer> q = q1;
q1 = q2;
q2 = q;
}
static void pop()
{
if (q1.isEmpty())
return ;
q1.remove();
curr_size--;
}
static int top()
{
if (q1.isEmpty())
return - 1 ;
return q1.peek();
}
static int size()
{
return curr_size;
}
}
public static void main(String[] args)
{
Stack s = new Stack();
s.push( 1 );
s.push( 2 );
s.push( 3 );
System.out.println( "current size: " + s.size());
System.out.println(s.top());
s.pop();
System.out.println(s.top());
s.pop();
System.out.println(s.top());
System.out.println( "current size: " + s.size());
}
}
Python3
from queue import Queue
class Stack:
def __init__( self ):
self .q1 = Queue()
self .q2 = Queue()
self .curr_size = 0
def push( self , x):
self .curr_size + = 1
self .q2.put(x)
while ( not self .q1.empty()):
self .q2.put( self .q1.queue[ 0 ])
self .q1.get()
self .q = self .q1
self .q1 = self .q2
self .q2 = self .q
def pop( self ):
if ( self .q1.empty()):
return
self .q1.get()
self .curr_size - = 1
def top( self ):
if ( self .q1.empty()):
return - 1
return self .q1.queue[ 0 ]
def size( self ):
return self .curr_size
if __name__ = = '__main__' :
s = Stack()
s.push( 1 )
s.push( 2 )
s.push( 3 )
print ( "current size: " , s.size())
print (s.top())
s.pop()
print (s.top())
s.pop()
print (s.top())
print ( "current size: " , s.size())
C#
using System;
using System.Collections;
class GfG {
public class Stack {
public Queue q1 = new Queue();
public Queue q2 = new Queue();
public int curr_size;
public Stack()
{
curr_size = 0;
}
public void push( int x)
{
curr_size++;
q2.Enqueue(x);
while (q1.Count > 0) {
q2.Enqueue(q1.Peek());
q1.Dequeue();
}
Queue q = q1;
q1 = q2;
q2 = q;
}
public void pop()
{
if (q1.Count == 0)
return ;
q1.Dequeue();
curr_size--;
}
public int top()
{
if (q1.Count == 0)
return -1;
return ( int )q1.Peek();
}
public int size()
{
return curr_size;
}
};
public static void Main(String[] args)
{
Stack s = new Stack();
s.push(1);
s.push(2);
s.push(3);
Console.WriteLine( "current size: " + s.size());
Console.WriteLine(s.top());
s.pop();
Console.WriteLine(s.top());
s.pop();
Console.WriteLine(s.top());
Console.WriteLine( "current size: " + s.size());
}
}
Output :
current size: 3 3 2 1 current size: 1
Method 2 (By making pop operation costly)
In push operation, the new element is always enqueued to q1. In pop() operation, if q2 is empty then all the elements except the last, are moved to q2. Finally the last element is dequeued from q1 and returned.
- push(s, x) operation:
- Enqueue x to q1 (assuming size of q1 is unlimited).
- pop(s) operation:
- One by one dequeue everything except the last element from q1 and enqueue to q2.
- Dequeue the last item of q1, the dequeued item is result, store it.
- Swap the names of q1 and q2
- Return the item stored in step 2.
C++
#include <bits/stdc++.h>
using namespace std;
class Stack {
queue< int > q1, q2;
int curr_size;
public :
Stack()
{
curr_size = 0;
}
void pop()
{
if (q1.empty())
return ;
while (q1.size() != 1) {
q2.push(q1.front());
q1.pop();
}
q1.pop();
curr_size--;
queue< int > q = q1;
q1 = q2;
q2 = q;
}
void push( int x)
{
q1.push(x);
curr_size++;
}
int top()
{
if (q1.empty())
return -1;
while (q1.size() != 1) {
q2.push(q1.front());
q1.pop();
}
int temp = q1.front();
q1.pop();
q2.push(temp);
queue< int > q = q1;
q1 = q2;
q2 = q;
return temp;
}
int size()
{
return curr_size;
}
};
int main()
{
Stack s;
s.push(1);
s.push(2);
s.push(3);
s.push(4);
cout << "current size: " << s.size()
<< endl;
cout << s.top() << endl;
s.pop();
cout << s.top() << endl;
s.pop();
cout << s.top() << endl;
cout << "current size: " << s.size()
<< endl;
return 0;
}
Java
import java.util.*;
class Stack {
Queue<Integer> q1 = new LinkedList<>(), q2 = new LinkedList<>();
int curr_size;
public Stack()
{
curr_size = 0 ;
}
void remove()
{
if (q1.isEmpty())
return ;
while (q1.size() != 1 ) {
q2.add(q1.peek());
q1.remove();
}
q1.remove();
curr_size--;
Queue<Integer> q = q1;
q1 = q2;
q2 = q;
}
void add( int x)
{
q1.add(x);
curr_size++;
}
int top()
{
if (q1.isEmpty())
return - 1 ;
while (q1.size() != 1 ) {
q2.add(q1.peek());
q1.remove();
}
int temp = q1.peek();
q1.remove();
q2.add(temp);
Queue<Integer> q = q1;
q1 = q2;
q2 = q;
return temp;
}
int size()
{
return curr_size;
}
public static void main(String[] args)
{
Stack s = new Stack();
s.add( 1 );
s.add( 2 );
s.add( 3 );
s.add( 4 );
System.out.println( "current size: " + s.size());
System.out.println(s.top());
s.remove();
System.out.println(s.top());
s.remove();
System.out.println(s.top());
System.out.println( "current size: " + s.size());
}
}
Python3
from queue import Queue
class Stack:
def __init__( self ):
self .q1 = Queue()
self .q2 = Queue()
self .curr_size = 0
def push( self , x):
self .q1.put(x)
self .curr_size + = 1
def pop( self ):
if ( self .q1.empty()):
return
while ( self .q1.qsize() ! = 1 ):
self .q2.put( self .q1.get())
popped = self .q1.get()
self .curr_size - = 1
self .q = self .q1
self .q1 = self .q2
self .q2 = self .q
def top( self ):
if ( self .q1.empty()):
return
while ( self .q1.qsize() ! = 1 ):
self .q2.put( self .q1.get())
top = self .q1.queue[ 0 ]
self .q2.put( self .q1.get())
self .q = self .q1
self .q1 = self .q2
self .q2 = self .q
return top
def size( self ):
return self .curr_size
if __name__ = = '__main__' :
s = Stack()
s.push( 1 )
s.push( 2 )
s.push( 3 )
s.push( 4 )
print ( "current size: " , s.size())
print (s.top())
s.pop()
print (s.top())
s.pop()
print (s.top())
print ( "current size: " , s.size())
C#
using System;
using System.Collections;
class GfG
{
public class Stack
{
public Queue q1 = new Queue();
public Queue q2 = new Queue();
public void Push( int x) => q1.Enqueue(x);
public int Pop()
{
if (q1.Count == 0)
return -1;
while (q1.Count > 1)
{
q2.Enqueue(q1.Dequeue());
}
int res = ( int )q1.Dequeue();
Queue temp = q1;
q1 = q2;
q2 = temp;
return res;
}
public int Size() => q1.Count;
public int Top()
{
if (q1.Count == 0)
return -1;
while (q1.Count > 1)
{
q2.Enqueue(q1.Dequeue());
}
int res = ( int )q1.Dequeue();
q2.Enqueue(res);
Queue temp = q1;
q1 = q2;
q2 = temp;
return res;
}
};
public static void Main(String[] args)
{
Stack s = new Stack();
s.Push(1);
Console.WriteLine( "Size of Stack: " + s.Size() + "\tTop : " + s.Top());
s.Push(7);
Console.WriteLine( "Size of Stack: " + s.Size() + "\tTop : " + s.Top());
s.Push(9);
Console.WriteLine( "Size of Stack: " + s.Size() + "\tTop : " + s.Top());
s.Pop();
Console.WriteLine( "Size of Stack: " + s.Size() + "\tTop : " + s.Top());
s.Pop();
Console.WriteLine( "Size of Stack: " + s.Size() + "\tTop : " + s.Top());
s.Push(5);
Console.WriteLine( "Size of Stack: " + s.Size() + "\tTop : " + s.Top());
}
}
Output :
current size: 4 4 3 2 current size: 2
References:
Implement Stack using Two Queues
This article is compiled by Sumit Jain and reviewed by GeeksforGeeks team. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.
Write the Implementation (.cpp File) of the Counter Class of the Previous Exercise.
Source: https://www.geeksforgeeks.org/implement-stack-using-queue/
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