Implement Stack using two Queues

 

For implementing stack, we require two stacks, although it can be implemented using a single queue, but here we are going to see how to do it using two queues.

Queue is the reverse of Stack. 

Why? - Queue works on FIFO(First In First Out), and Stack works on LIFO (Last In First Out).

How we are going to do it?  

1. Initialize two queues queue_1, and queue_2.

2. Define two functions - push(item) and pop().

3. In push, what we are doing? - 

• Append item in queue_2, then pop all elements from queue_1, and append to queue_2. 
• Now, pop all elements from queue_2, and append to queue_1

     queue_2 is queue now because first item inserted will pop first.

4. In pop, if queue_1 is empty, return -1 (means no item in the queue). Else, pop elements from queue_1, and return the last element of queue_1.

Code - 

queue_1 = []

queue_2 = []

def push(x):

         global queue_1

        global queue_2

        queue_2.append(x)

        while len(queue_1)!=0:

                   top = queue_1.pop()

                  queue_2.append(top)

        

      while len(queue_2)!=0:

                    temp = queue_2.pop()

                   queue_1.append(temp)

        

  def pop():

           global queue_1

           global queue_2

           temp = -1

           if len(queue_1)!=0:

                  temp = queue_1.pop()

           

           return temp

I hope you understood the implementation, if you have any doubt do comment, I will help you out.