Queue Using Two Stacks in DSA C# - Build from Scratch

Queue Using Two Stacks

📖 Scenario: Imagine you are managing a line of customers waiting for service. You want to keep track of the order they arrive and serve them in the same order. However, you only have two stacks (like two piles of plates) to help you manage this line.

🎯 Goal: You will build a queue using two stacks. This queue will allow adding customers to the end and serving customers from the front, using only stack operations.

📋 What You'll Learn

Create two stacks named stack1 and stack2 to hold integers representing customer IDs.

Create a variable n to represent the number of customers to add.

Write code to enqueue customers with IDs from 1 to n into stack1.

Write code to dequeue all customers in the correct order using stack1 and stack2.

Print the dequeued customer IDs in order, separated by spaces.

💡 Why This Matters

🌍 Real World

Queues are used in many places like customer service lines, print job management, and task scheduling. Using two stacks to build a queue helps understand how data structures can work together.

💼 Career

Understanding how to implement one data structure using others is a common interview question and helps improve problem-solving skills in software development.

1

Create Two Stacks

Create two stacks named stack1 and stack2 using Stack<int>.

DSA C#

// Your code here

Hint

Use Stack<int> to create two empty stacks named stack1 and stack2.

2

Add Number of Customers

Create an integer variable named n and set it to 5 to represent the number of customers.

DSA C#

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        Stack stack1 = new Stack();
        Stack stack2 = new Stack();
        // Your code here
    }
}

Hint

Set n to 5 to represent 5 customers.

3

Enqueue Customers into stack1

Use a for loop with variable i from 1 to n inclusive, and push i into stack1.

DSA C#

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        Stack stack1 = new Stack();
        Stack stack2 = new Stack();
        int n = 5;
        // Your code here
    }
}

Hint

Use a for loop to add customer IDs 1 to 5 into stack1 using Push.

4

Dequeue and Print Customers in Order

Move all items from stack1 to stack2 by popping from stack1 and pushing into stack2. Then pop each item from stack2 and print it separated by spaces using Console.Write.

DSA C#

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        Stack stack1 = new Stack();
        Stack stack2 = new Stack();
        int n = 5;
        for (int i = 1; i <= n; i++)
        {
            stack1.Push(i);
        }
        // Your code here
    }
}

Hint

Pop all items from stack1 and push them into stack2. Then pop from stack2 and print each item with a space.

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