Implement a stack using single queue
Last Updated :
19 Mar, 2025
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We are given a queue data structure, the task is to implement a stack using a single queue.
Also Read: Stack using two queues
The idea is to keep the newly inserted element always at the front of the queue, preserving the order of previous elements by appending the new element at the back and rotating the queue by size n so that the new item is at the front.
// Push an element x in the stack s
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.
// pop an item from stack s
pop(s)
1) Dequeue an item from q
#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;
}
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(s.top());
s.pop();
s.push(30);
s.pop();
System.out.println(s.top());
}
}
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(str(s[-1]));
s.pop();
s.append(30);
s.pop();
print(str(s[-1]));
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(s.top());
s.pop();
s.push(30);
s.pop();
Console.WriteLine(s.top());
}
}
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())
{
console.log("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);
console.log(Top());
Pop();
Push(30);
Pop();
console.log(Top());
Output
20 10
Time complexity
For Push Operations: O(n) as we need to rotate the queue to bring the newly added element to the front. ( where n is the size of the queue )
For Pop Operations: O(1)
Auxiliary Space: O(n)
