Hi,

How to implement a stack using two queues ? Help pls!

Regards,
Preethi

Think of it this way: For each new value that's pushed onto the 'stack', you pop all of the values from queue1 onto queue2, then push the new value onto queue1. Taking a simple example, say you have queue1 with a single value, 1, and you want to push 2 onto it so that the next pop gives you 2 instead of 1:

queue1: * 1
queue2: *

Empty queue1 onto queue2:

queue1: *
queue2: * 1

Push 2 onto queue 1:

queue1: * 2
queue2: * 1

Then empty queue2 onto queue1:

queue1: * 1 2
queue2: *

So now, by pushing 1 and then 2 onto the queue this way, popping them both off would give you 2 1, just like a stack, instead of 1 2, like a queue.

Hi,

Is there a mistake in the last step ?

The last step that is given is:
Then empty queue2 onto queue1:
queue1: * 1 2
queue2: *
I think the last step is :
queue1: * 2 1
queue2: *
Is it right ??

> Is it right ??
No, you're thinking that the left side of my queues designate the front, which wasn't the intention. It works like this in the example that I gave:

New values go in here -> 1 2 -> Old values come out here

If you want to think of it the other way around, that's okay too, just reverse the values.