Implement Stack Using Queue in DSA C# - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a queue for the stack implementation.

DSA C#

private Queue<int> [1] = new Queue<int>();

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

AqueueStack

BstackQueue

Cstack

Dqueue

Attempts:

3 left

💡 Hint

Common Mistakes

Using a generic name like 'queue' which is too vague.

Using 'stack' which is misleading because it's actually a queue.

The queue used to implement the stack is named 'stackQueue' to clearly represent its purpose.

2fill in blank

medium

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

DSA C#

public void Push(int x) {
    stackQueue.[1](x);
}

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

AAdd

BPush

CEnqueue

DInsert

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Push' which is not a queue method.

Using 'Add' which is not a method of Queue in C#.

To add an element to the queue, we use the Enqueue method.

3fill in blank

hard

Fix the error in the Pop method to remove the top element from the stack.

DSA C#

public int Pop() {
    int size = stackQueue.Count;
    for (int i = 0; i < size - 1; i++) {
        stackQueue.Enqueue(stackQueue.[1]());
    }
    return stackQueue.Dequeue();
}

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

APop

BPeek

CRemove

DDequeue

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Pop' which is not a queue method.

Using 'Peek' which only returns the front element without removing it.

To rotate the queue, we remove the front element using Dequeue and add it back to the end.

4fill in blank

hard

Fill both blanks to implement the Top method that returns the top element without removing it.

DSA C#

public int Top() {
    int size = stackQueue.Count;
    for (int i = 0; i < size - 1; i++) {
        stackQueue.Enqueue(stackQueue.[1]());
    }
    int topElement = stackQueue.[2]();
    stackQueue.Enqueue(topElement);
    return topElement;
}

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

ADequeue

BPeek

CPop

DRemove

Attempts:

3 left

💡 Hint

Common Mistakes

Using Pop which is not a queue method.

Using Dequeue twice which removes the top element permanently.

We remove elements with Dequeue to rotate, then use Peek to get the top element without removing it permanently.

5fill in blank

hard

Fill both blanks to implement the Empty method that checks if the stack is empty.

DSA C#

public bool Empty() {
    return stackQueue.[1] == [2];
}

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

ACount

B0

CLength

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Using Length which is not a property of Queue in C#.

Comparing to null which is not correct for Count.

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

Practice

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

easy

A. Use a linked list directly to implement stack

B. Use one queue and reorder elements to simulate stack behavior

C. Use two stacks to simulate queue behavior

D. Use an array to simulate queue behavior

Solution

1. Step 1: Understand stack and queue behavior

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

2. Step 2: Simulate stack using queue

   By reordering elements in a single queue after each push, the newest element is always at the front, simulating stack pop behavior.

3. Final Answer:

   Use one queue and reorder elements to simulate stack behavior -> Option B

4. Quick Check:

   Stack via queue = reorder elements [OK]

Hint: Remember stack is LIFO, queue is FIFO; reorder queue to mimic stack [OK]

Common Mistakes:

• Confusing stack and queue order
• Using two stacks instead of queues
• Ignoring element reordering after push

2. Which of the following is the correct C# syntax to declare a queue of integers?

easy

A. Queue<int> queue = new Queue<int>();

B. Queue queue = new Queue<int>();

C. Queue<int> queue = Queue<int>();

D. Queue<int> queue = new Queue();

Solution

1. Step 1: Review C# generic queue declaration

   In C#, a generic queue is declared as Queue<T> with new keyword and parentheses.

2. Step 2: Check each option

   Queue<int> queue = new Queue<int>(); correctly uses Queue<int> queue = new Queue<int>(); which is valid syntax.

3. Final Answer:

   Queue<int> queue = new Queue<int>(); -> Option A

4. Quick Check:

   Generic queue syntax = Queue<int> queue = new Queue<int>(); [OK]

Hint: Use new Queue<int>() to declare generic queue in C# [OK]

Common Mistakes:

• Omitting new keyword
• Missing generic type in constructor
• Using non-generic Queue without type

3. What will be the output of the following C# code implementing stack using one queue?

var stack = new StackUsingQueue();
stack.Push(10);
stack.Push(20);
stack.Push(30);
Console.WriteLine(stack.Pop());
Console.WriteLine(stack.Pop());

medium

A. 10\n30

B. 10\n20

C. 20\n30

D. 30\n20

Solution

1. Step 1: Understand push operation in stack using queue

   Each push adds element and reorders queue so last pushed is at front.

2. Step 2: Trace pop operations

   First pop returns 30 (last pushed), second pop returns 20 (second last pushed).

3. Final Answer:

   30\n20 -> Option D

4. Quick Check:

   Stack pop order = last in first out = 30, then 20 [OK]

Hint: Pop returns last pushed element due to queue reorder [OK]

Common Mistakes:

• Assuming queue FIFO order for pop
• Mixing push and pop order
• Ignoring reorder after push

4. Identify the potential issue in this C# method for stack push using one queue:

public void Push(int x) {
    queue.Enqueue(x);
    for (int i = 0; i < queue.Count - 1; i++) {
        queue.Enqueue(queue.Dequeue());
    }
}

medium

A. Enqueue should be Dequeue

B. Missing return statement

C. Using queue.Count in loop condition changes during iteration

D. Loop should run queue.Count times

Solution

1. Step 1: Analyze loop condition with queue.Count

   queue.Count changes momentarily during iteration (decreases after Dequeue, restores after Enqueue).

2. Step 2: Explain why this is a concern

   Although the size restores before the next condition check and works correctly, using a changing value in the loop condition can be confusing and is bad practice. Store count beforehand: int n = queue.Count; for(int i=0; i < n-1; i++)...

3. Final Answer:

   Using queue.Count in loop condition changes during iteration -> Option C

4. Quick Check:

   Loop count must be stored before loop [OK]

Hint: Store queue.Count before loop to avoid changing condition [OK]

Common Mistakes:

• Using dynamic queue.Count in loop condition
• Not rotating queue elements correctly
• Confusing enqueue and dequeue operations

5. You want to implement a stack using two queues. Which approach correctly simulates the stack's pop operation?

hard

A. Push elements into queue1; for pop, move all but last from queue1 to queue2, dequeue last from queue1, then swap queues

B. Push elements into queue2; for pop, dequeue directly from queue2

C. Push elements into queue1; pop by dequeuing from queue1 without moving elements

D. Push elements into queue2; for pop, move all elements to queue1 and dequeue first

Solution

1. Step 1: Understand two-queue stack implementation

   One queue holds elements; pop requires removing last pushed element, which is at the end.

2. Step 2: Describe correct pop operation

   Move all but last element from queue1 to queue2, dequeue last from queue1 (stack pop), then swap queues for next operations.

3. Final Answer:

   Push elements into queue1; for pop, move all but last from queue1 to queue2, dequeue last from queue1, then swap queues -> Option A

4. Quick Check:

   Pop = move all but last, dequeue last, swap queues [OK]

Hint: Pop by moving all but last element, then dequeue last [OK]

Common Mistakes:

• Dequeuing directly without moving elements
• Not swapping queues after pop
• Confusing which queue holds elements