We are given queue data structure, the task is to implement stack using only given queue data structure.
We have discussed a solution that uses two queues. In this article, a new solution is discussed that uses only one queue. This solution assumes that we can find size of queue at any point. The idea is to keep newly inserted element always at front of queue, keeping order of previous elements same.
Below are complete steps.
// x is the element to be pushed and s is stack push(s, x) 1) Let size of q be s. 1) Enqueue x to q 2) One by one Dequeue s items from queue and enqueue them. // Removes an item from stack pop(s) 1) Dequeue an item from q
Below is implementation of the idea.
C++
// C++ program to implement a stack using // single queue #include<bits/stdc++.h> using namespace std;
// User defined stack that uses a queue class Stack
{ queue< int >q;
public :
void push( int val);
void pop();
int top();
bool empty();
}; // Push operation void Stack::push( int val)
{ // Get previous size of queue
int s = q.size();
// Push current element
q.push(val);
// Pop (or Dequeue) all previous
// elements and put them after current
// element
for ( int i=0; i<s; i++)
{
// this will add front element into
// rear of queue
q.push(q.front());
// this will delete front element
q.pop();
}
} // Removes the top element void Stack::pop()
{ if (q.empty())
cout << "No elements\n" ;
else
q.pop();
} // Returns top of stack int Stack::top()
{ return (q.empty())? -1 : q.front();
} // Returns true if Stack is empty else false bool Stack::empty()
{ return (q.empty());
} // Driver code int main()
{ Stack s;
s.push(10);
s.push(20);
cout << s.top() << endl;
s.pop();
s.push(30);
s.pop();
cout << s.top() << endl;
return 0;
} |
Java
// Java program to implement stack using a // single queue import java.util.LinkedList;
import java.util.Queue;
public class stack
{ Queue<Integer> q = new LinkedList<Integer>();
// Push operation
void push( int val)
{
// get previous size of queue
int size = q.size();
// Add current element
q.add(val);
// Pop (or Dequeue) all previous
// elements and put them after current
// element
for ( int i = 0 ; i < size; i++)
{
// this will add front element into
// rear of queue
int x = q.remove();
q.add(x);
}
}
// Removes the top element
int pop()
{
if (q.isEmpty())
{
System.out.println( "No elements" );
return - 1 ;
}
int x = q.remove();
return x;
}
// Returns top of stack
int top()
{
if (q.isEmpty())
return - 1 ;
return q.peek();
}
// Returns true if Stack is empty else false
boolean isEmpty()
{
return q.isEmpty();
}
// Driver program to test above methods
public static void main(String[] args)
{
stack s = new stack();
s.push( 10 );
s.push( 20 );
System.out.println( "Top element :" + s.top());
s.pop();
s.push( 30 );
s.pop();
System.out.println( "Top element :" + s.top());
}
} // This code is contributed by Rishabh Mahrsee |
Python3
# Python3 program to implement stack using a # single queue q = []
# append operation def append(val):
# get previous size of queue
size = len (q)
# Add current element
q.append(val);
# Pop (or Dequeue) all previous
# elements and put them after current
# element
for i in range (size):
# this will add front element into
# rear of queue
x = q.pop( 0 );
q.append(x);
# Removes the top element def pop():
if ( len (q) = = 0 ):
print ( "No elements" );
return - 1 ;
x = q.pop( 0 );
return x;
# Returns top of stack def top():
if ( len (q) = = 0 ):
return - 1 ;
return q[ - 1 ]
# Returns true if Stack is empty else false def isEmpty():
return len (q) = = 0 ;
# Driver program to test above methods if __name__ = = '__main__' :
s = []
s.append( 10 );
s.append( 20 );
print ( "Top element :" + str (s[ - 1 ]));
s.pop();
s.append( 30 );
s.pop();
print ( "Top element :" + str (s[ - 1 ]));
# This code is contributed by rutvik_56.
|
C#
// C# program to implement stack using a // single queue using System;
using System.Collections.Generic;
public class stack
{ Queue< int > q = new Queue< int >();
// Push operation
void push( int val)
{
// get previous size of queue
int size = q.Count;
// Add current element
q.Enqueue(val);
// Pop (or Dequeue) all previous
// elements and put them after current
// element
for ( int i = 0; i < size; i++)
{
// this will add front element into
// rear of queue
int x = q.Dequeue();
q.Enqueue(x);
}
}
// Removes the top element
int pop()
{
if (q.Count == 0)
{
Console.WriteLine( "No elements" );
return -1;
}
int x = q.Dequeue();
return x;
}
// Returns top of stack
int top()
{
if (q.Count == 0)
return -1;
return q.Peek();
}
// Returns true if Stack is empty else false
bool isEmpty()
{
if (q.Count == 0)
return true ;
return false ;
}
// Driver program to test above methods
public static void Main(String[] args)
{
stack s = new stack();
s.push(10);
s.push(20);
Console.WriteLine( "Top element :" + s.top());
s.pop();
s.push(30);
s.pop();
Console.WriteLine( "Top element :" + s.top());
}
} // This code has been contributed by Rajput-Ji |
Javascript
<script> // Javascript program to implement stack using a single queue
let q = [];
// Push operation
function Push(val)
{
// get previous size of queue
let Size = q.length;
// Add current element
q.push(val);
// Pop (or Dequeue) all previous
// elements and put them after current
// element
for (let i = 0; i < Size; i++)
{
// this will add front element into
// rear of queue
let x = q[0];
q.shift();
q.push(x);
}
}
// Removes the top element
function Pop()
{
if (isEmpty())
{
document.write( "No elements" + "</br>" );
return -1;
}
let x = q[0];
q.shift();
return x;
}
// Returns top of stack
function Top()
{
if (isEmpty())
return -1;
return q[0];
}
// Returns true if Stack is empty else false
function isEmpty()
{
if (q.length == 0)
return true ;
return false ;
}
Push(10);
Push(20);
document.write(Top() + "</br>" );
Pop();
Push(30);
Pop();
document.write(Top() + "</br>" );
// This code is contributed by decode2207. </script> |
Output
20 10
Time complexity: O(N) where N is size of stack
Auxiliary Space: O(N)
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