# 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<``int``> 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<``int``> 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[0])
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[0]

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<``int``> 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<``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(); ` `        ``}  ` `         `  `        ``// 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<``int``> 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