Interview Cake

You only have free questions left (including this one).

But it doesn't have to end here! Sign up for the 7-day coding interview crash course and you'll get a free Interview Cake problem every week.

Implement a queue with 2 stacks. Your queue should have an enqueue and a dequeue method and it should be "first in first out" (FIFO).

Optimize for the time cost of m calls on your queue. These can be any mix of enqueue and dequeue calls.

Assume you already have a stack implementation and it gives time push and pop.

We can get runtime for m calls. Crazy, right?

Start your free trial!

Log in or sign up with one click to get immediate access to free mock interview questions

Where do I enter my password?

Actually, we don't support password-based login. Never have. Just the OAuth methods above. Why?

  1. It's easy and quick. No "reset password" flow. No password to forget.
  2. It lets us avoid storing passwords that hackers could access and use to try to log into our users' email or bank accounts.
  3. It makes it harder for one person to share a paid Interview Cake account with multiple people.

Start your free trial!

Log in or sign up with one click to get immediate access to free mock interview questions

Where do I enter my password?

Actually, we don't support password-based login. Never have. Just the OAuth methods above. Why?

  1. It's easy and quick. No "reset password" flow. No password to forget.
  2. It lets us avoid storing passwords that hackers could access and use to try to log into our users' email or bank accounts.
  3. It makes it harder for one person to share a paid Interview Cake account with multiple people.

Each enqueue is clearly time, and so is each dequeue when outStack_ has items. Dequeue on an empty outStack_ is order of the number of items in inStack_ at that moment, which can vary significantly.

Notice that the more expensive a dequeue on an empty outStack_ is (that is, the more items we have to move from inStack_ to outStack_), the more -time dequeues off of a non-empty outStack_ it wins us in the future. Once items are moved from inStack_ to outStack_ they just sit there, ready to be dequeued in O(1) time. An item never moves "backwards" in our data structure.

We might guess that this "averages out" so that in a set of m enqueues and dequeues the total cost of all dequeues is actually just . To check this rigorously, we can use the accounting method, counting the time cost per item instead of per enqueue or dequeue.

So let's look at the worst case for a single item, which is the case where it is enqueued and then later dequeued. In this case, the item enters inStack_ (costing 1 push), then later moves to outStack_ (costing 1 pop and 1 push), then later comes off outStack_ to get returned (costing 1 pop).

Each of these 4 pushes and pops is time. So our total cost per item is . Our m enqueue and dequeue operations put m or fewer items into the system, giving a total runtime of .

Start your free trial!

Log in or sign up with one click to get immediate access to free mock interview questions

Where do I enter my password?

Actually, we don't support password-based login. Never have. Just the OAuth methods above. Why?

  1. It's easy and quick. No "reset password" flow. No password to forget.
  2. It lets us avoid storing passwords that hackers could access and use to try to log into our users' email or bank accounts.
  3. It makes it harder for one person to share a paid Interview Cake account with multiple people.

Reset editor

Powered by qualified.io

. . .