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. 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) // x is the element to be pushed and s is stack
1) Enqueue x to q2
2) One by one dequeue everything from q1 and enqueue to q2.
3) Swap the names of q1 and q2
// Swapping of names is done to avoid one more movement of all elements
// from q2 to q1.

pop(s)
1) Dequeue an item from q1 and return it.

C++

 /* Program to implement a stack using  two queue */ #include    using namespace std;    class Stack {      // Two inbuilt queues     queue q1, q2;            // To maintain current number of     // elements     int curr_size;        public:     Stack()     {         curr_size = 0;     }        void push(int x)     {         curr_size++;            // Push x first in empty q2         q2.push(x);            // Push all the remaining          // elements in q1 to q2.          while (!q1.empty())         {             q2.push(q1.front());             q1.pop();         }            // swap the names of two queues         queue q = q1;         q1 = q2;         q2 = q;     }        void pop(){            // if no elements are there in q1          if (q1.empty())             return ;         q1.pop();         curr_size--;     }        int top()     {         if (q1.empty())             return -1;         return q1.front();     }        int size()     {         return curr_size;     } };    // driver code 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; } // This code is contributed by Chhavi

Java

 /* Java Program to implement a stack using  two queue */ import java.util.*;    class GfG {    static class Stack  {      // Two inbuilt queues      static Queue q1 = new LinkedList();     static Queue q2 = new LinkedList();             // To maintain current number of      // elements      static int curr_size;         Stack()      {          curr_size = 0;      }         static void push(int x)      {          curr_size++;             // Push x first in empty q2          q2.add(x);             // Push all the remaining          // elements in q1 to q2.          while (!q1.isEmpty())          {              q2.add(q1.peek());              q1.remove();          }             // swap the names of two queues          Queue q = q1;          q1 = q2;          q2 = q;      }         static void pop(){             // if no elements are there in q1          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;      }  };     // driver code  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());  } }  // This code is contributed by Prerna

Python3

# Program to implement a stack using
# two queue
from queue import Queue

class Stack:

def __init__(self):

# Two inbuilt queues
self.q1 = Queue()
self.q2 = Queue()

# To maintain current number
# of elements
self.curr_size = 0

def push(self, x):
self.curr_size += 1

# Push x first in empty q2
self.q2.put(x)

# Push all the remaining
# elements in q1 to q2.
while (not self.q1.empty()):
self.q2.put(self.q1.queue)
self.q1.get()

# swap the names of two queues
self.q = self.q1
self.q1 = self.q2
self.q2 = self.q

def pop(self):

# if no elements are there in q1
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

def size(self):
return self.curr_size

# Driver Code
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())

# This code is contributed by PranchalK

C#

 /* C# Program to implement a stack using  two queue */ using System; using System.Collections;    class GfG  {    public class Stack  {      // Two inbuilt queues      public Queue q1 = new Queue();     public Queue q2 = new Queue();             // To maintain current number of      // elements      public int curr_size;         public Stack()      {          curr_size = 0;      }         public void push(int x)      {          curr_size++;             // Push x first in empty q2          q2.Enqueue(x);             // Push all the remaining          // elements in q1 to q2.          while (q1.Count > 0)          {              q2.Enqueue(q1.Peek());              q1.Dequeue();          }             // swap the names of two queues          Queue q = q1;          q1 = q2;          q2 = q;      }         public void pop()     {             // if no elements are there in q1          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;      }  };     // Driver code  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());  } }     // This code is contributed by Arnab Kundu

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)
1) Enqueue x to q1 (assuming size of q1 is unlimited).

pop(s)
1) One by one dequeue everything except the last element from q1 and enqueue to q2.
2) Dequeue the last item of q1, the dequeued item is result, store it.
3) Swap the names of q1 and q2
4) Return the item stored in step 2.
// Swapping of names is done to avoid one more movement of all elements
// from q2 to q1.
 /* Program to implement a stack  using two queue */ #include using namespace std;    class Stack {     queue q1, q2;     int curr_size;        public:     Stack()     {         curr_size = 0;     }        void pop()     {         if (q1.empty())             return;            // Leave one element in q1 and          // push others in q2.         while (q1.size() != 1)         {             q2.push(q1.front());             q1.pop();         }            // Pop the only left element          // from q1         q1.pop();         curr_size--;            // swap the names of two queues         queue 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();         }                     // last pushed element         int temp = q1.front();                    // to empty the auxiliary queue after         // last operation         q1.pop();                 // push last element to q2         q2.push(temp);            // swap the two queues names         queue q = q1;         q1 = q2;         q2 = q;         return temp;     }        int size()     {         return curr_size;     } };    // Driver code 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; } // This code is contributed by Chhavi

Output :

current size: 4
4
3
2
current size: 2

References:
Implement Stack using Two Queues