Queue Using Two Stacks in DSA C# - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Enqueue and Dequeue Operations

What is the printed state of the queue after performing these operations?

Enqueue 10, Enqueue 20, Dequeue, Enqueue 30, Dequeue

DSA C#

class QueueUsingStacks {
    private Stack<int> stack1 = new Stack<int>();
    private Stack<int> stack2 = new Stack<int>();

    public void Enqueue(int x) {
        stack1.Push(x);
    }

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

    public string PrintQueue() {
        var temp = new List<int>();
        foreach (var item in stack2) temp.Add(item);
        var s1 = new List<int>();
        foreach (var item in stack1) s1.Add(item);
        s1.Reverse();
        temp.AddRange(s1);
        return string.Join(" -> ", temp) + " -> null";
    }
}

var q = new QueueUsingStacks();
q.Enqueue(10);
q.Enqueue(20);
q.Dequeue();
q.Enqueue(30);
q.Dequeue();
Console.WriteLine(q.PrintQueue());

A null

B 20 -> 30 -> null

C 10 -> 30 -> null

D 30 -> null

💡 Hint

Remember that dequeue moves elements from stack1 to stack2 only when stack2 is empty.

After first dequeue, 10 is removed leaving 20 in stack2. Then enqueue 30 adds to stack1. Second dequeue removes 20 directly from stack2. Remaining element is 30.

🧠 Conceptual

intermediate

Understanding Stack Transfers in Queue Implementation

Why do we transfer elements from stack1 to stack2 only when stack2 is empty during dequeue operation in a queue implemented using two stacks?

A To reverse the order of elements so the oldest element is on top for dequeue.

B To clear stack1 to save memory.

C To sort the elements in ascending order before dequeue.

D To duplicate elements for backup.

💡 Hint

Think about how stacks work and how queue order is maintained.

Stack reverses order when elements are popped and pushed to another stack. This reversal ensures the first inserted element is dequeued first.

🔧 Debug

advanced

Identify the Error in Dequeue Method

What error will occur when calling Dequeue on an empty queue implemented using two stacks with this code snippet?

DSA C#

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

A NullReferenceException because stack2 is null.

B InvalidOperationException due to popping from an empty stack.

C IndexOutOfRangeException because of invalid index access.

D No error, returns default int value 0.

💡 Hint

What happens if both stacks are empty and you try to pop?

If both stacks are empty, stack2.Pop() tries to remove an element from an empty stack, causing InvalidOperationException.

❓ Predict Output

advanced

Final Queue State After Mixed Operations

What is the printed queue state after these operations?

Enqueue 5, Enqueue 15, Enqueue 25, Dequeue, Enqueue 35, Dequeue, Enqueue 45

DSA C#

class QueueUsingStacks {
    private Stack<int> stack1 = new Stack<int>();
    private Stack<int> stack2 = new Stack<int>();

    public void Enqueue(int x) {
        stack1.Push(x);
    }

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

    public string PrintQueue() {
        var temp = new List<int>();
        foreach (var item in stack2) temp.Add(item);
        var s1 = new List<int>();
        foreach (var item in stack1) s1.Add(item);
        s1.Reverse();
        temp.AddRange(s1);
        return string.Join(" -> ", temp) + " -> null";
    }
}

var q = new QueueUsingStacks();
q.Enqueue(5);
q.Enqueue(15);
q.Enqueue(25);
q.Dequeue();
q.Enqueue(35);
q.Dequeue();
q.Enqueue(45);
Console.WriteLine(q.PrintQueue());

A 35 -> 45 -> null

B 15 -> 25 -> 35 -> 45 -> null

C 25 -> 35 -> 45 -> null

D 5 -> 15 -> 25 -> 35 -> 45 -> null

💡 Hint

Track elements moved between stacks carefully after each dequeue.

First dequeue removes 5, second dequeue removes 15. Remaining elements are 25, 35, 45 in order.

🚀 Application

expert

Time Complexity Analysis of Queue Using Two Stacks

What is the amortized time complexity of the Dequeue operation in a queue implemented using two stacks?

A O(1) amortized time per dequeue operation

B O(n) time for every dequeue operation

C O(log n) time per dequeue operation

D O(n^2) time in worst case

💡 Hint

Consider how many times each element is moved between stacks.

Each element is pushed and popped at most twice, so average cost per dequeue is constant time.

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