Implement Stack Using Queue in DSA Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main challenge when implementing a stack using queues?

The main challenge is to simulate Last-In-First-Out (LIFO) behavior using First-In-First-Out (FIFO) queues.

intermediate

Describe the push operation in a stack implemented using a single queue.

To push, enqueue the new element, then rotate the queue by dequeuing and enqueuing all elements before the new element to bring it to the front.

beginner

How does the pop operation work in a stack implemented using a queue?

Pop is done by dequeuing the front element of the queue, which represents the top of the stack after rotation.

intermediate

Why do we rotate the queue after pushing a new element in the stack implementation?

Rotation moves the newly added element to the front, ensuring it behaves like the top of the stack for subsequent pop operations.

intermediate

What is the time complexity of push and pop operations in a stack implemented using a single queue?

Push operation takes O(n) time due to rotation, while pop operation takes O(1) time as it just dequeues the front element.

Which data structure behavior does a stack follow?

A Last-In-First-Out (LIFO)

B First-In-First-Out (FIFO)

C Random Access

D Priority Based

A stack follows Last-In-First-Out (LIFO) behavior, meaning the last element added is the first to be removed.

In implementing a stack using a single queue, what operation is needed after enqueueing a new element?

A Clear the queue

B Dequeue all elements

C Sort the queue

D Rotate the queue to bring the new element to the front

After enqueueing, rotating the queue moves the new element to the front to simulate stack behavior.

What is the time complexity of the pop operation in a stack implemented using a single queue?

A O(n)

B O(1)

C O(log n)

D O(n^2)

Pop operation is O(1) because it just dequeues the front element of the queue.

Which of these is NOT a valid operation for a stack?

A Enqueue

B Pop

C Peek

D Push

Enqueue is an operation for queues, not stacks.

Why do we use a queue to implement a stack?

A Because stacks are deprecated

B Because queues are faster

C To practice converting FIFO to LIFO behavior

D To reduce memory usage

Implementing a stack using a queue helps understand how to simulate LIFO behavior using FIFO data structures.

Explain how to implement the push operation of a stack using a single queue.

Think about how to make the newest element accessible first.

Describe the difference in time complexity between push and pop operations in a stack implemented using a single queue.

Consider what happens after adding a new element.

Practice

1. What is the main idea behind implementing a stack using a queue?

easy

A. Use a queue to sort elements in ascending order

B. Use two stacks to implement a queue

C. Use a stack to implement queue operations

D. Use one or two queues to simulate stack operations like push and pop

Solution

1. Step 1: Understand stack and queue operations

   A stack follows Last-In-First-Out (LIFO) and a queue follows First-In-First-Out (FIFO).

2. Step 2: Simulate stack using queue(s)

   By using one or two queues cleverly, we can reorder elements to behave like a stack.

3. Final Answer:

   Use one or two queues to simulate stack operations like push and pop -> Option D

4. Quick Check:

   Stack via queue = simulate LIFO with FIFO [OK]

Hint: Remember stack is LIFO, queue is FIFO; simulate LIFO with queue(s) [OK]

Common Mistakes:

• Confusing stack and queue order
• Thinking queue sorts elements automatically
• Using stack to implement queue instead

2. Which of the following TypeScript code snippets correctly declares a queue using an array?

easy

A. const queue: number[] = [];

B. let queue = new Stack<number>();

C. const queue = {};

D. let queue: Queue = new Array();

Solution

1. Step 1: Check correct TypeScript array declaration

   To declare a queue as an array of numbers, use const queue: number[] = [];

2. Step 2: Identify incorrect declarations

   let queue = new Stack<number>(); uses Stack, not queue; C is an empty object; A uses undefined type Queue.

3. Final Answer:

   const queue: number[] = []; -> Option A

4. Quick Check:

   Array queue declaration = const queue: number[] = [] [OK]

Hint: Use array with type annotation for queue: const queue: number[] = [] [OK]

Common Mistakes:

• Using Stack instead of array for queue
• Declaring queue as empty object {}
• Using undefined types without import

3. Given the following TypeScript code implementing a stack using one queue, what will be the output after the operations?

class StackUsingQueue {
  private queue: number[] = [];

  push(x: number) {
    this.queue.push(x);
    for (let i = 0; i < this.queue.length - 1; i++) {
      this.queue.push(this.queue.shift()!);
    }
  }

  pop(): number | undefined {
    return this.queue.shift();
  }
}

const stack = new StackUsingQueue();
stack.push(1);
stack.push(2);
stack.push(3);
console.log(stack.pop());
console.log(stack.pop());

medium

A. 3
2

B. 1
2

C. 2
3

D. Error at runtime

Solution

1. Step 1: Trace push operations

   Push 1: queue=[1]
   Push 2: queue=[2,1] after rotation
   Push 3: queue=[3,2,1] after rotation

2. Step 2: Trace pop operations

   Pop returns queue.shift(): 3, then next pop returns 2.

3. Final Answer:

   3 2 -> Option A

4. Quick Check:

   Stack pop order = 3 then 2 [OK]

Hint: Push rotates queue to front; pop removes front element [OK]

Common Mistakes:

• Not rotating queue after push
• Assuming pop removes last element of array
• Confusing shift() with pop()

4. Identify the bug in this TypeScript code for stack using two queues and suggest the fix:

class StackUsingTwoQueues {
  private q1: number[] = [];
  private q2: number[] = [];

  push(x: number) {
    this.q1.push(x);
  }

  pop(): number | undefined {
    while (this.q1.length > 1) {
      this.q2.push(this.q1.shift()!);
    }
    return this.q1.shift();
  }
}

medium

A. pop() should return q2.shift() instead of q1.shift()

B. push() should push to q2 instead of q1

C. pop() does not swap q1 and q2 after popping; add swap after pop

D. No bug; code works correctly

Solution

1. Step 1: Analyze pop method behavior

   Pop moves all but last element from q1 to q2, then removes last from q1.

2. Step 2: Identify missing queue swap

   After pop, q1 is empty and q2 has elements, but q1 and q2 are not swapped, so next push/pop fail.

3. Final Answer:

   pop() does not swap q1 and q2 after popping; add swap after pop -> Option C

4. Quick Check:

   Swap queues after pop to maintain correct state [OK]

Hint: Remember to swap queues after pop in two-queue stack [OK]

Common Mistakes:

• Forgetting to swap queues after pop
• Pushing to wrong queue
• Returning wrong queue element

5. You want to implement a stack using a single queue in TypeScript. Which approach correctly ensures that the pop() operation returns the last pushed element efficiently?

class StackUsingOneQueue {
  private queue: number[] = [];

  push(x: number) {
    this.queue.push(x);
    // What should be done here?
  }

  pop(): number | undefined {
    return this.queue.shift();
  }
}

hard

A. After push, sort the queue in descending order

B. After push, rotate the queue by moving all elements except the last to the back

C. After push, reverse the queue array

D. No extra operation needed after push

Solution

1. Step 1: Understand stack behavior with one queue

   To simulate LIFO, after pushing, rotate queue so new element is at front.

2. Step 2: Rotate queue after push

   Move all elements except last pushed to back, so pop (shift) removes last pushed.

3. Final Answer:

   After push, rotate the queue by moving all elements except the last to the back -> Option B

4. Quick Check:

   Rotate queue after push to simulate stack pop [OK]

Hint: Rotate queue after push to keep last element at front [OK]

Common Mistakes:

• Sorting queue instead of rotating
• Reversing queue array (inefficient)
• Not rotating queue after push