Implement Stack Using Queue in DSA Typescript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add an element to the stack using a queue.

DSA Typescript

push(x: number): void {
  this.queue.push([1]);
}

Drag options to blanks, or click blank then click option'

Athis.queue.pop()

Bthis.queue.shift()

Cundefined

Dx

Attempts:

3 left

💡 Hint

Common Mistakes

Using shift() or pop() instead of adding the new element.

Passing undefined instead of the element.

To push an element onto the stack, we add it to the queue using push(x).

2fill in blank

medium

Complete the code to remove the top element from the stack using the queue.

DSA Typescript

pop(): number | undefined {
  for (let i = 0; i < this.queue.length - 1; i++) {
    this.queue.push(this.queue.[1]());
  }
  return this.queue.shift();
}

Drag options to blanks, or click blank then click option'

Ashift

Bunshift

Cpush

Dpop

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop() which removes the last element instead of the first.

Using push() or unshift() which add elements instead of removing.

We use shift() to remove the front element and push it back to rotate the queue until the last element remains.

3fill in blank

hard

Fix the error in the peek method to return the top element without removing it.

DSA Typescript

peek(): number | undefined {
  for (let i = 0; i < this.queue.length - 1; i++) {
    this.queue.push(this.queue.shift());
  }
  const top = this.queue.[1]();
  this.queue.push(this.queue.shift());
  return top;
}

Drag options to blanks, or click blank then click option'

Ashift

Bpop

Cpush

Dunshift

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop() which removes the last element.

Using push() or unshift() which add elements.

We use shift() to get the front element without removing it permanently by pushing it back immediately.

4fill in blank

hard

Fill both blanks to check if the stack is empty.

DSA Typescript

isEmpty(): boolean {
  return this.queue.[1] === [2];
}

Drag options to blanks, or click blank then click option'

Alength

Bsize

C0

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Using size which is not a property of arrays in TypeScript.

Comparing length to null instead of 0.

We check if the queue's length is 0 to determine if the stack is empty.

5fill in blank

hard

Fill all three blanks to implement the push method that rotates the queue to simulate stack behavior.

DSA Typescript

push(x: number): void {
  this.queue.push([1]);
  for (let i = 0; i < this.queue.[2] - 1; i++) {
    this.queue.push(this.queue.[3]());
  }
}

Drag options to blanks, or click blank then click option'

Ax

Blength

Cshift

Dpop

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop() instead of shift() for rotation.

Using wrong property instead of length.

Not rotating the queue after push.

We add the new element x, then rotate the queue by removing from front (shift) and adding to back to keep the last pushed element at front.

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