LinkedList usage in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of LinkedList AddFirst and AddLast

What is the output of the following C# code?

C Sharp (C#)

using System;
using System.Collections.Generic;

class Program {
    static void Main() {
        LinkedList<int> list = new LinkedList<int>();
        list.AddFirst(10);
        list.AddLast(20);
        list.AddFirst(5);
        foreach (var item in list) {
            Console.Write(item + " ");
        }
    }
}

A 10 5 20

B 20 10 5

C 10 20 5

D 5 10 20

💡 Hint

Remember AddFirst adds to the start, AddLast adds to the end.

AddFirst(10) puts 10 at start. AddLast(20) adds 20 at end. AddFirst(5) adds 5 at start, so list is 5, 10, 20.

❓ Predict Output

intermediate

LinkedList Remove and Count

What is the output of this C# code snippet?

C Sharp (C#)

using System;
using System.Collections.Generic;

class Program {
    static void Main() {
        LinkedList<string> names = new LinkedList<string>();
        names.AddLast("Anna");
        names.AddLast("Bob");
        names.AddLast("Cara");
        names.Remove("Bob");
        Console.WriteLine(names.Count);
    }
}

A 2

B 1

C 0

D 3

💡 Hint

Removing an element decreases the count by one.

Initially 3 elements. Removing "Bob" leaves 2 elements: "Anna" and "Cara".

🔧 Debug

advanced

Find the error in LinkedList node insertion

What error will this C# code produce when run?

C Sharp (C#)

using System;
using System.Collections.Generic;

class Program {
    static void Main() {
        LinkedList<int> numbers = new LinkedList<int>();
        numbers.AddLast(1);
        numbers.AddLast(3);
        var node = numbers.Find(2);
        numbers.AddAfter(node, 2);
        foreach (var n in numbers) {
            Console.Write(n + " ");
        }
    }
}

A NullReferenceException

B InvalidOperationException

C No error, output: 1 3 2

D ArgumentNullException

💡 Hint

Check what happens if Find returns null and you use it as a node.

Find(2) returns null because 2 is not in list. AddAfter(null, 2) throws NullReferenceException because node is null.

❓ Predict Output

advanced

LinkedList Reverse Traversal Output

What is the output of this C# code?

C Sharp (C#)

using System;
using System.Collections.Generic;

class Program {
    static void Main() {
        LinkedList<char> letters = new LinkedList<char>();
        letters.AddLast('a');
        letters.AddLast('b');
        letters.AddLast('c');
        var node = letters.Last;
        while (node != null) {
            Console.Write(node.Value);
            node = node.Previous;
        }
    }
}

A abc

B cba

C bac

D cab

💡 Hint

Start from the last node and move backwards.

Starting at 'c' (last), then 'b', then 'a', prints 'cba'.

🧠 Conceptual

expert

LinkedList Node Removal Impact

Given a LinkedList with elements 1, 2, 3, 4, 5, what is the value of the node after removing the node with value 3?

C Sharp (C#)

using System;
using System.Collections.Generic;

class Program {
    static void Main() {
        LinkedList<int> list = new LinkedList<int>(new int[] {1, 2, 3, 4, 5});
        var node = list.Find(3);
        var nextNode = node.Next;
        list.Remove(node);
        Console.WriteLine(nextNode.Value);
    }
}

A 3

B 5

C 4

D Throws InvalidOperationException

💡 Hint

Check what happens to the Next property of a node after removal.

nextNode points to node with value 4 before removal. Removing node 3 does not change nextNode reference. So nextNode.Value is 4.

Practice

1. What is a key characteristic of a LinkedList in C#?

easy

A. It stores elements in nodes linked by references.

B. It stores elements in a fixed-size array.

C. It only allows adding elements at the end.

D. It cannot remove elements once added.

Solution

1. Step 1: Understand LinkedList structure

   A LinkedList stores elements in nodes, where each node points to the next (and possibly previous) node.

2. Step 2: Compare options with LinkedList behavior

   Only It stores elements in nodes linked by references. correctly describes this linked node structure; others describe arrays or incorrect behaviors.

3. Final Answer:

   It stores elements in nodes linked by references. -> Option A

4. Quick Check:

   LinkedList = nodes linked by references [OK]

Hint: LinkedList uses nodes connected by links, not arrays. [OK]

Common Mistakes:

• Thinking LinkedList uses arrays internally
• Assuming LinkedList only adds at the end
• Believing LinkedList cannot remove elements

2. Which of the following is the correct way to add an element at the start of a LinkedList<int> named list?

easy

A. list.AddStart(10);

B. list.AddFirst(10);

C. list.InsertAt(0, 10);

D. list.PushFront(10);

Solution

1. Step 1: Recall LinkedList method names

   The method to add an element at the start is AddFirst.

2. Step 2: Check each option's validity

   Only AddFirst is a valid LinkedList method; others are invalid or do not exist.

3. Final Answer:

   list.AddFirst(10); -> Option B

4. Quick Check:

   AddFirst adds at start [OK]

Hint: Use AddFirst to add at the start of LinkedList. [OK]

Common Mistakes:

• Using non-existent methods like AddStart or PushFront
• Confusing LinkedList with List methods
• Trying to use InsertAt which LinkedList does not have

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

var list = new LinkedList<string>();
list.AddLast("apple");
list.AddFirst("banana");
list.AddLast("cherry");
foreach(var item in list) Console.Write(item + " ");

medium

A. banana apple cherry

B. apple banana cherry

C. cherry apple banana

D. banana cherry apple

Solution

1. Step 1: Track insertion order

   First, "apple" is added last, so list: apple. Then "banana" added first, so list: banana, apple. Then "cherry" added last, so list: banana, apple, cherry.

2. Step 2: Understand foreach iteration order

   Foreach iterates from first to last node, so output is "banana apple cherry ".

3. Final Answer:

   banana apple cherry -> Option A

4. Quick Check:

   First = banana, last = cherry [OK]

Hint: AddFirst puts item at start; AddLast at end. [OK]

Common Mistakes:

• Assuming AddLast adds at start
• Confusing order of AddFirst and AddLast
• Expecting output in reverse order

4. Identify the error in this code snippet:

var list = new LinkedList<int>();
list.AddFirst(1);
list.AddLast(2);
list.Remove(3);
Console.WriteLine(list.Count);

medium

A. Remove(3) throws an exception because 3 is not in the list.

B. Count property does not exist on LinkedList.

C. AddFirst and AddLast methods are invalid for LinkedList.

D. Remove(3) does nothing since 3 is not found; Count remains 2.

Solution

1. Step 1: Understand Remove behavior

   Remove(value) tries to remove the first node with that value. If not found, it does nothing and returns false; no exception is thrown.

2. Step 2: Check Count after removal attempt

   Since 3 is not in the list, list remains with 2 elements; Count is 2.

3. Final Answer:

   Remove(3) does nothing since 3 is not found; Count remains 2. -> Option D

4. Quick Check:

   Remove missing value = no error, Count unchanged [OK]

Hint: Remove missing item does not throw error, just returns false. [OK]

Common Mistakes:

• Expecting Remove to throw exception if item missing
• Thinking AddFirst/AddLast are invalid
• Assuming Count is not a property

5. Given a LinkedList<int> with values 1, 2, 3, 4, 5, which code snippet correctly removes all even numbers from the list?

hard

A. foreach(var node in list) { if(node % 2 == 0) list.Remove(node); }

B. for(int i = 0; i < list.Count; i++) { if(list.ElementAt(i) % 2 == 0) list.Remove(list.ElementAt(i)); }

C. var current = list.First; while(current != null) { var next = current.Next; if(current.Value % 2 == 0) list.Remove(current); current = next; }

D. list.RemoveAll(x => x % 2 == 0);

Solution

1. Step 1: Understand safe removal during iteration

   Removing nodes while iterating requires storing next node before removal to avoid invalid references.

2. Step 2: Analyze each option

   foreach(var node in list) { if(node % 2 == 0) list.Remove(node); } uses foreach which throws error on modification during iteration. var current = list.First; while(current != null) { var next = current.Next; if(current.Value % 2 == 0) list.Remove(current); current = next; } correctly uses a while loop with next node saved. for(int i = 0; i < list.Count; i++) { if(list.ElementAt(i) % 2 == 0) list.Remove(list.ElementAt(i)); } uses ElementAt which is inefficient and unsafe. list.RemoveAll(x => x % 2 == 0); is invalid as LinkedList has no RemoveAll method.

3. Final Answer:

   var current = list.First; while(current != null) { var next = current.Next; if(current.Value % 2 == 0) list.Remove(current); current = next; } -> Option C

4. Quick Check:

   Use while loop with next saved to remove nodes safely [OK]

Hint: Save next node before removal to avoid iteration errors. [OK]

Common Mistakes:

• Modifying list inside foreach causes runtime error
• Using RemoveAll which LinkedList does not have
• Using ElementAt which is inefficient and unsafe