Search for a Value in Linked List in DSA C# - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output after searching for value 3?

Given the linked list and search code below, what will be printed when searching for value 3?

DSA C#

public class Node {
    public int data;
    public Node next;
    public Node(int d) { data = d; next = null; }
}

public class LinkedList {
    public Node head;

    public bool Search(int key) {
        Node current = head;
        while (current != null) {
            if (current.data == key) return true;
            current = current.next;
        }
        return false;
    }
}

LinkedList list = new LinkedList();
list.head = new Node(1);
list.head.next = new Node(2);
list.head.next.next = new Node(3);
list.head.next.next.next = new Node(4);

bool found = list.Search(3);
System.Console.WriteLine(found ? "Found" : "Not Found");

A Runtime error

B "Not Found"

C "Found"

D Compilation error

💡 Hint

Think about how the search method checks each node's data.

The search method traverses the linked list and returns true when it finds the node with data 3. Since 3 is in the list, it prints "Found".

❓ Predict Output

intermediate

What is the output after searching for value 5?

Given the linked list and search code below, what will be printed when searching for value 5?

DSA C#

public class Node {
    public int data;
    public Node next;
    public Node(int d) { data = d; next = null; }
}

public class LinkedList {
    public Node head;

    public bool Search(int key) {
        Node current = head;
        while (current != null) {
            if (current.data == key) return true;
            current = current.next;
        }
        return false;
    }
}

LinkedList list = new LinkedList();
list.head = new Node(1);
list.head.next = new Node(2);
list.head.next.next = new Node(3);
list.head.next.next.next = new Node(4);

bool found = list.Search(5);
System.Console.WriteLine(found ? "Found" : "Not Found");

A "Not Found"

B "Found"

C NullReferenceException

D Compilation error

💡 Hint

Check if the value 5 exists in the linked list nodes.

The search method returns false because the value 5 is not present in the linked list. So it prints "Not Found".

🔧 Debug

advanced

What error does this search code produce?

Consider this search method in a linked list. What error will it cause when called?

DSA C#

public bool Search(int key) {
    Node current = head;
    while (current.next != null) {
        if (current.data == key) return true;
        current = current.next;
    }
    return false;
}

A No error, works correctly

B NullReferenceException

C StackOverflowException

D Compilation error

💡 Hint

Think about what happens when current is the last node.

The loop condition uses current.next != null, so when current is the last node, current.next is null and the loop stops before checking the last node's data. If the key is in the last node, it will not be found. Also, if the list is empty (head is null), current.next causes NullReferenceException.

🧠 Conceptual

advanced

How many nodes are checked in the search method?

In the search method below, how many nodes are checked when searching for a value in a linked list of 5 nodes?

DSA C#

public bool Search(int key) {
    Node current = head;
    while (current != null) {
        if (current.data == key) return true;
        current = current.next;
    }
    return false;
}

A Up to 4 nodes

B Exactly 5 nodes always

C Only 1 node

D Up to 5 nodes

💡 Hint

The loop stops when the value is found or all nodes are checked.

The search checks nodes one by one until it finds the value or reaches the end. So it can check from 1 up to all 5 nodes depending on where the value is.

🚀 Application

expert

What is the output after searching for value 7 in this linked list?

Given the linked list with nodes 10 -> 20 -> 30 -> 40 -> 50, and the search method below, what will be printed when searching for value 7?

DSA C#

public class Node {
    public int data;
    public Node next;
    public Node(int d) { data = d; next = null; }
}

public class LinkedList {
    public Node head;

    public bool Search(int key) {
        Node current = head;
        while (current != null) {
            if (current.data == key) return true;
            current = current.next;
        }
        return false;
    }
}

LinkedList list = new LinkedList();
list.head = new Node(10);
list.head.next = new Node(20);
list.head.next.next = new Node(30);
list.head.next.next.next = new Node(40);
list.head.next.next.next.next = new Node(50);

bool found = list.Search(7);
System.Console.WriteLine(found ? "Found" : "Not Found");

A "Not Found"

B "Found"

C NullReferenceException

D Compilation error

💡 Hint

Check if 7 is in the list nodes.

The value 7 is not in the linked list nodes, so the search returns false and prints "Not Found".

Practice

1. What is the main purpose of searching for a value in a linked list?

easy

A. To add a new node at the end

B. To find if the value exists in the list

C. To delete the first node

D. To reverse the linked list

Solution

1. Step 1: Understand the search operation

   Searching means checking each node to see if it contains the target value.

2. Step 2: Identify the goal of searching

   The goal is to find whether the value exists anywhere in the linked list.

3. Final Answer:

   To find if the value exists in the list -> Option B

4. Quick Check:

   Search = Find value presence [OK]

Hint: Search means checking nodes for the value [OK]

Common Mistakes:

• Confusing search with insertion or deletion
• Thinking search changes the list
• Assuming search returns the node position always

2. Which of the following is the correct C# syntax to start traversing a linked list from its head node named head?

easy

A. Node current => head;

B. Node current == head;

C. Node current = head;

D. Node current <- head;

Solution

1. Step 1: Understand variable assignment syntax in C#

   To assign a reference, use single equals sign =.

2. Step 2: Check each option for correct assignment

   Node current = head; uses =, which is correct. Others use invalid operators.

3. Final Answer:

   Node current = head; -> Option C

4. Quick Check:

   Assignment uses = in C# [OK]

Hint: Use single = for assignment in C# [OK]

Common Mistakes:

• Using == instead of = for assignment
• Using invalid operators like => or <-
• Confusing assignment with comparison

3. What will be the output of the following C# code snippet?

class Node {
  public int data;
  public Node next;
  public Node(int d) { data = d; next = null; }
}

Node head = new Node(5);
head.next = new Node(10);
head.next.next = new Node(15);

Node current = head;
bool found = false;
while (current != null) {
  if (current.data == 10) {
    found = true;
    break;
  }
  current = current.next;
}
Console.WriteLine(found);

medium

A. NullReferenceException

B. False

C. Compilation error

D. True

Solution

1. Step 1: Trace the linked list nodes and search loop

   The list has nodes with data 5 -> 10 -> 15. The loop checks each node's data for 10.

2. Step 2: Check when the value 10 is found

   At second node, data is 10, so found becomes true and loop breaks.

3. Final Answer:

   True -> Option D

4. Quick Check:

   Value 10 found = True [OK]

Hint: Stop loop when value found, print true [OK]

Common Mistakes:

• Forgetting to break loop after finding value
• Printing current.data instead of found
• Null pointer errors if next not checked

4. Identify the error in the following code that searches for a value in a linked list:

Node current = head;
while (current.next != null) {
  if (current.data == 20) {
    Console.WriteLine("Found");
    break;
  }
  current = current.next;
}

medium

A. It skips checking the last node

B. It causes infinite loop

C. It throws NullReferenceException immediately

D. It never enters the loop

Solution

1. Step 1: Analyze the loop condition

   The loop runs while current.next != null, so it stops before last node.

2. Step 2: Understand impact on search

   The last node's data is never checked, so search may miss the value if it's there.

3. Final Answer:

   It skips checking the last node -> Option A

4. Quick Check:

   Loop condition misses last node [OK]

Hint: Use current != null to check all nodes [OK]

Common Mistakes:

• Using current.next != null instead of current != null
• Assuming last node is checked automatically
• Not handling empty list (head null)

5. Given a linked list where nodes may contain duplicate values, how can you modify the search to return the count of occurrences of a target value x instead of just checking presence?

hard

A. Traverse all nodes, increment count when node.data == x, then return count

B. Stop at first occurrence and return 1

C. Use recursion to find only the last occurrence

D. Delete nodes with value x and return total nodes left

Solution

1. Step 1: Understand the need to count all occurrences

   Since duplicates exist, we must check every node to count all matches.

2. Step 2: Traverse entire list and count matches

   Loop through each node, increment a counter when node.data equals x, then return the count.

3. Final Answer:

   Traverse all nodes, increment count when node.data == x, then return count -> Option A

4. Quick Check:

   Count all matches by full traversal [OK]

Hint: Count matches by checking every node [OK]

Common Mistakes:

• Stopping search after first match
• Using recursion incorrectly for counting
• Modifying list during search unintentionally