Traversal Forward and Backward in DSA C# - Build from Scratch

Traversal Forward and Backward

📖 Scenario: You have a list of daily temperatures recorded in a week. You want to look at these temperatures both from the start to the end of the week and then from the end back to the start.

🎯 Goal: Build a program that stores the temperatures in a list, then prints them forward (start to end) and backward (end to start) using simple loops.

📋 What You'll Learn

Create a list of integers called temperatures with exact values for each day.

Create an integer variable called count to hold the number of temperature entries.

Use a for loop with variable i to traverse temperatures forward and print each temperature.

Use a for loop with variable j to traverse temperatures backward and print each temperature.

💡 Why This Matters

🌍 Real World

Looking at data forward and backward is common in time series analysis, like checking temperatures, stock prices, or sensor readings.

💼 Career

Understanding how to traverse data structures in both directions is a basic skill for software developers working with lists, arrays, or other collections.

1

Create the temperature list

Create a list of integers called temperatures with these exact values: 23, 25, 22, 20, 18, 21, 19.

DSA C#

# Your code here

Hint

Use List<int> and initialize it with the exact numbers inside curly braces.

2

Add a count variable

Create an integer variable called count and set it to the number of items in the temperatures list.

DSA C#

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        List temperatures = new List { 23, 25, 22, 20, 18, 21, 19 };
        // Create count variable here
        
    }
}

Hint

Use the Count property of the list to get the number of elements.

3

Traverse forward and backward

Use a for loop with variable i from 0 to count - 1 to print each temperature forward. Then use a for loop with variable j from count - 1 down to 0 to print each temperature backward.

DSA C#

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        List temperatures = new List { 23, 25, 22, 20, 18, 21, 19 };
        int count = temperatures.Count;
        // Print forward
        
        // Print backward
    }
}

Hint

Use two separate for loops: one counting up from 0, the other counting down from count - 1.

4

Print the traversal results

Run the program to print the temperatures forward and backward as two separate lists.

DSA C#

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        List temperatures = new List { 23, 25, 22, 20, 18, 21, 19 };
        int count = temperatures.Count;

        // Print forward
        for (int i = 0; i = 0; j--)
        {
            Console.WriteLine(temperatures[j]);
        }
    }
}

Hint

Check the console output matches the forward list first, then the backward list.

Practice

1. What does Traversal Forward and Backward mean in the context of a doubly linked list?

easy

A. Visiting nodes from the start to the end and then from the end back to the start

B. Only visiting nodes from the start to the end

C. Visiting nodes randomly without order

D. Visiting nodes only from the end to the start

Solution

1. Step 1: Understand traversal directions

   Traversal forward means visiting nodes from the first node to the last node. Traversal backward means visiting nodes from the last node back to the first node.

2. Step 2: Apply to doubly linked list

   A doubly linked list allows moving in both directions because each node has links to the next and previous nodes.

3. Final Answer:

   Visiting nodes from the start to the end and then from the end back to the start -> Option A

4. Quick Check:

   Forward and backward traversal = visiting nodes both ways [OK]

Hint: Think of walking forward then turning back [OK]

Common Mistakes:

• Confusing traversal with random access
• Thinking traversal is only forward
• Ignoring backward links in doubly linked list

2. Which of the following is the correct syntax to traverse a doubly linked list forward in C#?

easy

A. for(Node current = tail; current != null; current = current.Next) { /* process current */ }

B. for(Node current = head; current != null; current = current.Next) { /* process current */ }

C. for(Node current = head; current != null; current = current.Prev) { /* process current */ }

D. for(Node current = tail; current != null; current = current.Prev) { /* process current */ }

Solution

1. Step 1: Identify forward traversal

   Forward traversal starts at the head and moves to the next node repeatedly until null.

2. Step 2: Check syntax correctness

   for(Node current = head; current != null; current = current.Next) { /* process current */ } correctly starts at head and moves using current = current.Next. Other options either start at tail or move backward.

3. Final Answer:

   for(Node current = head; current != null; current = current.Next) { /* process current */ } -> Option B

4. Quick Check:

   Forward traversal uses Next pointer from head [OK]

Hint: Forward means start at head and use Next [OK]

Common Mistakes:

• Starting at tail for forward traversal
• Using Prev pointer for forward traversal
• Confusing Next and Prev pointers

3. Given the doubly linked list with nodes containing values 1, 2, 3, 4, what is the output of this forward traversal code?

Node current = head;
while(current != null) {
    Console.Write(current.Value + " -> ");
    current = current.Next;
}
Console.WriteLine("null");

medium

A. null

B. 4 -> 3 -> 2 -> 1 -> null

C. 1 -> 2 -> 3 -> 4 -> null

D. 1 -> 2 -> 3 -> null

Solution

1. Step 1: Understand the list structure

   The list nodes are 1 linked to 2, then 3, then 4, in order.

2. Step 2: Trace the forward traversal

   Starting at head (1), print 1, move to 2, print 2, move to 3, print 3, move to 4, print 4, then current becomes null and loop ends.

3. Final Answer:

   1 -> 2 -> 3 -> 4 -> null -> Option C

4. Quick Check:

   Forward traversal prints nodes in order [OK]

Hint: Follow Next pointers from head to null [OK]

Common Mistakes:

• Printing backward order instead of forward
• Missing last node in output
• Printing null before nodes

4. Identify the error in this backward traversal code of a doubly linked list:

Node current = head;
while(current != null) {
    Console.Write(current.Value + " <- ");
    current = current.Prev;
}
Console.WriteLine("null");

medium

A. Missing null check in loop

B. Using Prev pointer instead of Next

C. Incorrect print format

D. Starting traversal from head instead of tail

Solution

1. Step 1: Understand backward traversal

   Backward traversal should start from the tail node, not the head.

2. Step 2: Analyze given code

   The code starts at head and tries to move backward using Prev, but head.Prev is null, so loop ends immediately.

3. Final Answer:

   Starting traversal from head instead of tail -> Option D

4. Quick Check:

   Backward traversal must start at tail [OK]

Hint: Backward means start at tail, not head [OK]

Common Mistakes:

• Starting backward traversal at head
• Confusing Prev and Next pointers
• Ignoring list ends (null pointers)

5. You have a doubly linked list with nodes: 10 <-> 20 <-> 30 <-> 40. Write a method to print the list forward and then backward without using extra data structures. What will be the output?

hard

A. 10 -> 20 -> 30 -> 40 -> null
40 -> 30 -> 20 -> 10 -> null

B. 40 -> 30 -> 20 -> 10 -> null
10 -> 20 -> 30 -> 40 -> null

C. 10 -> 20 -> 30 -> 40 -> null
null

D. null
40 -> 30 -> 20 -> 10 -> null

Solution

1. Step 1: Traverse forward from head to tail

   Start at head (10), print each node's value moving via Next until null: 10 -> 20 -> 30 -> 40 -> null.

2. Step 2: Traverse backward from tail to head

   After reaching tail (40), move backward using Prev pointer, printing: 40 -> 30 -> 20 -> 10 -> null.

3. Final Answer:

   10 -> 20 -> 30 -> 40 -> null 40 -> 30 -> 20 -> 10 -> null -> Option A

4. Quick Check:

   Forward then backward traversal prints nodes in both directions [OK]

Hint: Print forward using Next, backward using Prev [OK]

Common Mistakes:

• Trying to traverse backward from head
• Printing null before nodes
• Using extra data structures unnecessarily