Stack • Algorithms

Implement Stack using Queues

How can you mimic the Last-In-First-Out (LIFO) behavior of a Stack using only First-In-First-Out (FIFO) Queues?

The Single Queue Approach

You actually only need ONE queue! When you push an element, enqueue it to the back. Then, immediately dequeue all the previous elements and enqueue them back into the same queue.

Push - O(N) Time

Because you must rotate `size - 1` elements every time you push, the push operation becomes O(N). This effectively reverses the queue every time a new element arrives, keeping the newest element at the front.

Pop / Peek - O(1) Time

Because the push operation did all the heavy lifting to keep the newest element at the front, popping and peeking are extremely fast. Just `q.dequeue()` or `q.front()`.

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Stack • Algorithms

Implement Stack using Queues

How can you mimic the Last-In-First-Out (LIFO) behavior of a Stack using only First-In-First-Out (FIFO) Queues?

The Single Queue Approach

You actually only need ONE queue! When you push an element, enqueue it to the back. Then, immediately dequeue all the previous elements and enqueue them back into the same queue.

Push - O(N) Time

Pop / Peek - O(1) Time

Because the push operation did all the heavy lifting to keep the newest element at the front, popping and peeking are extremely fast. Just `q.dequeue()` or `q.front()`.

Stack Using Queue

Stack Using Queue

Implement Stack using Queues

The Single Queue Approach

Push - O(N) Time

Pop / Peek - O(1) Time

Implement Stack using Queues

The Single Queue Approach

Push - O(N) Time

Pop / Peek - O(1) Time