Queue Using Two Stacks in DSA C# - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to push an element into the input stack.

DSA C#

public void Enqueue(int x) {
    inputStack.[1](x);
}

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

APush

BPop

CPeek

DClear

Attempts:

3 left

💡 Hint

Common Mistakes

Using Pop instead of Push to add elements.

Using Peek which only views the top element.

To add an element to the queue, we push it onto the input stack using Push method.

2fill in blank

medium

Complete the code to check if the output stack is empty before transferring elements.

DSA C#

if (outputStack.[1] == 0) {
    while (inputStack.Count > 0) {
        outputStack.Push(inputStack.Pop());
    }
}

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

APeek

BLength

CEmpty

DCount

Attempts:

3 left

💡 Hint

Common Mistakes

Using Peek instead of Count to check emptiness.

Using a non-existent property like Empty.

We check the Count property to see if the output stack is empty (Count == 0).

3fill in blank

hard

Fix the error in the Dequeue method to return the correct element.

DSA C#

public int Dequeue() {
    if (outputStack.Count == 0) {
        while (inputStack.Count > 0) {
            outputStack.Push(inputStack.Pop());
        }
    }
    return outputStack.[1]();
}

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

APeek

BPop

CPush

DCount

Attempts:

3 left

💡 Hint

Common Mistakes

Using Peek which does not remove the element.

Using Push or Count which are incorrect here.

Dequeue removes and returns the front element, so we use Pop on outputStack.

4fill in blank

hard

Fill both blanks to implement the Peek method correctly.

DSA C#

public int Peek() {
    if (outputStack.[1] == 0) {
        while (inputStack.Count > 0) {
            outputStack.[2](inputStack.Pop());
        }
    }
    return outputStack.Peek();
}

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

ACount

BPop

CPush

DPeek

Attempts:

3 left

💡 Hint

Common Mistakes

Using Peek instead of Count to check emptiness.

Using Pop instead of Push to add elements.

We check if outputStack is empty by Count, then transfer elements using Push.

5fill in blank

hard

Fill all three blanks to implement the IsEmpty method correctly.

DSA C#

public bool IsEmpty() {
    return inputStack.[1] == 0 && outputStack.[2] == 0;
}

// Usage example:
// bool empty = queue.[3]();

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

ACount

BPeek

CIsEmpty

DLength

Attempts:

3 left

💡 Hint

Common Mistakes

Using Peek instead of Count to check emptiness.

Using a non-existent property or method.

We check if both stacks have zero Count to confirm the queue is empty, then call IsEmpty method.

Practice

1. What is the main idea behind implementing a Queue using two stacks?

easy

A. Use one stack for enqueue and another for dequeue operations

B. Use two stacks to store elements in sorted order

C. Use two stacks to reverse the order twice to maintain queue order

D. Use two stacks to store only even and odd elements separately

Solution

1. Step 1: Understand queue order

   A queue follows First-In-First-Out (FIFO) order, meaning the first element added is the first removed.

2. Step 2: Use two stacks to reverse order twice

   One stack reverses the order when pushing elements, and the second stack reverses it back when popping, maintaining FIFO.

3. Final Answer:

   Use two stacks to reverse the order twice to maintain queue order -> Option C

4. Quick Check:

   Two reversals keep FIFO order [OK]

Hint: Two stacks reverse order twice to simulate queue [OK]

Common Mistakes:

• Thinking stacks store sorted elements
• Using stacks for separate data types
• Confusing enqueue and dequeue roles

2. Which of the following is the correct syntax to push an element 5 into a stack named stack1 in C#?

easy

A. stack1.Push(5);

B. stack1.Add(5);

C. stack1.Insert(5);

D. stack1.Enqueue(5);

Solution

1. Step 1: Recall stack push method

   In C#, the Stack<T> class uses the Push method to add elements.

2. Step 2: Check method names

   Add and Insert are not stack methods; Enqueue is for queues.

3. Final Answer:

   stack1.Push(5); -> Option A

4. Quick Check:

   Stack push method = Push() [OK]

Hint: Use Push() to add elements to stack [OK]

Common Mistakes:

• Using Add() or Insert() which are not stack methods
• Confusing Enqueue() with Push()
• Using wrong method names

3. What will be the output of the following C# code implementing a queue using two stacks?

var queue = new QueueUsingTwoStacks();
queue.Enqueue(10);
queue.Enqueue(20);
queue.Enqueue(30);
Console.WriteLine(queue.Dequeue());
queue.Enqueue(40);
Console.WriteLine(queue.Dequeue());

medium

A. 30\n40

B. 10\n20

C. 20\n30

D. 10\n30

Solution

1. Step 1: Enqueue elements 10, 20, 30

   Elements are pushed into the first stack in order: 10, then 20, then 30.

2. Step 2: Dequeue operation

   On dequeue, elements are popped from the second stack after transferring from the first, so 10 is dequeued first.

3. Step 3: Enqueue 40 and dequeue again

   40 is pushed into the first stack. Next dequeue returns 20, the next in queue order.

4. Final Answer:

   10 20 -> Option B

5. Quick Check:

   FIFO order maintained: 10 then 20 [OK]

Hint: Dequeue returns oldest enqueued element first [OK]

Common Mistakes:

• Assuming dequeue returns last pushed element
• Not transferring elements between stacks correctly
• Mixing enqueue and dequeue order

4. Identify the error in the following C# code snippet for dequeue operation in a queue using two stacks:

public int Dequeue()
{
    if (stack2.Count == 0)
    {
        while (stack1.Count > 0)
            stack2.Push(stack1.Pop());
    }
    return stack1.Pop();
}

medium

A. Should pop from stack2, not stack1, after transfer

B. Should check if stack1 is empty before popping

C. Should push elements back to stack1 after popping

D. Should use Peek() instead of Pop()

Solution

1. Step 1: Understand dequeue logic

   After transferring elements from stack1 to stack2, dequeue should pop from stack2 to maintain correct order.

2. Step 2: Identify wrong pop call

   The code incorrectly pops from stack1 instead of stack2 after transfer.

3. Final Answer:

   Should pop from stack2, not stack1, after transfer -> Option A

4. Quick Check:

   Pop from stack2 after transfer [OK]

Hint: Pop from second stack after transfer for dequeue [OK]

Common Mistakes:

• Popping from wrong stack after transfer
• Not checking if stacks are empty
• Using Peek() instead of Pop() for dequeue

5. You want to implement a queue using two stacks where Enqueue is O(1) time and Dequeue is amortized O(1). Which approach is best?

hard

A. Push elements to stack2 on enqueue; pop from stack1 on dequeue

B. Push elements alternately to stack1 and stack2 on enqueue; pop from stack1 on dequeue

C. Push elements to stack1 on enqueue; pop from stack1 on dequeue without transfer

D. Push all elements to stack1 on enqueue; transfer to stack2 only when stack2 is empty during dequeue

Solution

1. Step 1: Understand enqueue O(1) requirement

   Enqueue should just push to stack1 without extra operations for O(1) time.

2. Step 2: Understand dequeue amortized O(1)

   Dequeue transfers elements from stack1 to stack2 only when stack2 is empty, making transfers infrequent and amortized O(1).

3. Step 3: Evaluate options

   Push all elements to stack1 on enqueue; transfer to stack2 only when stack2 is empty during dequeue matches this approach; others do not maintain correct order or time complexity.

4. Final Answer:

   Push all elements to stack1 on enqueue; transfer to stack2 only when stack2 is empty during dequeue -> Option D

5. Quick Check:

   Lazy transfer on dequeue gives amortized O(1) [OK]

Hint: Transfer only when needed on dequeue for amortized O(1) [OK]

Common Mistakes:

• Transferring on every enqueue causing O(n) enqueue
• Popping from wrong stack without transfer
• Alternating pushes breaking queue order