Implement a stack using single queue

04 Jul, 2022
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++
Java
Python3
C#
Javascript
// C++ program to implement a stack using// single queue#include<bits/stdc++.h>using namespace std; // User defined stack that uses a queueclass Stack{    queue<int>q;public:    void push(int val);    void pop();    int top();    bool empty();}; // Push operationvoid 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 elementvoid Stack::pop(){    if (q.empty())        cout << "No elements\n";    else        q.pop();} // Returns top of stackint  Stack::top(){    return (q.empty())? -1 : q.front();} // Returns true if Stack is empty else falsebool Stack::empty(){    return (q.empty());} // Driver codeint 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 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 program to implement stack using a# single queue  q = [] # append operationdef 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 elementdef pop():     if (len(q) == 0):         print("No elements");        return -1;         x = q.pop(0);    return x; # Returns top of stackdef top():     if(len(q) == 0):        return -1;    return q[-1] # Returns true if Stack is empty else falsedef isEmpty():     return len(q)==0; # Driver program to test above methodsif __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# program to implement stack using a// single queueusing 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
<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>
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