Hash Map vs Array vs Linked List for Lookup in DSA C# - Interactive Comparison Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to access the element at index 2 in the array.

DSA C#

int[] numbers = {1, 2, 3, 4, 5};
int value = numbers[[1]];

Drag options to blanks, or click blank then click option'

A2

B3

C1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using 3 as index which is out of range for the third element.

The array index starts at 0, so index 2 accesses the third element which is 3.

2fill in blank

medium

Complete the code to check if a key exists in the dictionary.

DSA C#

Dictionary<string, int> ages = new Dictionary<string, int> { {"Alice", 30}, {"Bob", 25} };
bool hasKey = ages.[1]("Alice");

Drag options to blanks, or click blank then click option'

AHasKey

BContainsKey

CContainsValue

DExists

Attempts:

3 left

💡 Hint

Common Mistakes

Using ContainsValue which checks values, not keys.

Use ContainsKey to check if a dictionary has a specific key.

3fill in blank

hard

Fix the error in the linked list traversal to print all values.

DSA C#

LinkedList<int> list = new LinkedList<int>(new int[] {10, 20, 30});
LinkedListNode<int> current = list.First;
while (current != null) {
    Console.WriteLine(current.Value);
    current = current.[1];
}

Drag options to blanks, or click blank then click option'

APrevious

BNextNode

Cnext

DNext

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase 'next' which does not exist.

The correct property to move to the next node is Next with uppercase 'N'.

4fill in blank

hard

Fill both blanks to populate a dictionary that includes only keys with values greater than 10.

DSA C#

var filtered = new Dictionary<string, int>();
foreach (var kvp in data) {
    if (kvp.Value [1] 10) {
        filtered.[2](kvp.Key, kvp.Value);
    }
}

Drag options to blanks, or click blank then click option'

A>

BAdd

CAddRange

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' which filters wrong values.

Using AddRange which is not a dictionary method.

Use '>' to check if value is greater than 10 and 'Add' to add key-value pairs to the dictionary.

5fill in blank

hard

Fill all three blanks to create a dictionary from a list where keys are the words and values are their lengths, but only include words longer than 3 characters.

DSA C#

var words = new List<string> {"cat", "house", "dog", "elephant"};
var result = new Dictionary<string, int>();
foreach (var word in words) {
    if (word.[2] [3] 3) {
        result.Add([1], word.[2]);
    }
}

Drag options to blanks, or click blank then click option'

Aword

BLength

C>

DCount

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Count' which is not a string property.

Using '<' which filters wrong words.

Use 'word' as key, 'Length' property for value, and '>' to filter words longer than 3 characters.

Practice

1. Which data structure provides the fastest average lookup time for a key-value pair?

easy

A. Stack

B. Array

C. Linked List

D. Hash Map

Solution

1. Step 1: Understand lookup times for each data structure

   Hash Map uses hashing to find keys quickly, averaging O(1) time. Arrays require searching or indexing, O(1) if index known but O(n) if searching by value. Linked Lists require traversal, O(n) time.

2. Step 2: Compare average lookup times

   Hash Map is fastest on average for key-value lookup due to direct hashing. Arrays and Linked Lists are slower for key-based lookup.

3. Final Answer:

   Hash Map -> Option D

4. Quick Check:

   Fastest lookup = Hash Map [OK]

Hint: Hash Map uses hashing for O(1) average lookup [OK]

Common Mistakes:

• Confusing array index lookup with key lookup
• Assuming linked list lookup is fast
• Ignoring average vs worst-case lookup times

2. Which of the following is the correct way to declare a HashMap (Dictionary) in C#?

easy

A. var map = new Dictionary();

B. var map = new HashMap();

C. var map = new Map();

D. var map = new Array();

Solution

1. Step 1: Recall C# syntax for dictionary declaration

   C# uses Dictionary class for hash maps, declared as new Dictionary().

2. Step 2: Check each option for correctness

   var map = new Dictionary(); uses Dictionary correctly. Options A and C use non-existent classes HashMap and Map. var map = new Array(); uses Array incorrectly.

3. Final Answer:

   var map = new Dictionary(); -> Option A

4. Quick Check:

   C# Dictionary syntax = var map = new Dictionary(); [OK]

Hint: Use Dictionary for hash maps in C# [OK]

Common Mistakes:

• Using Java or other language classes like HashMap
• Confusing Map with Dictionary
• Trying to use Array for key-value pairs

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

var list = new LinkedList<int>();
list.AddLast(10);
list.AddLast(20);
list.AddLast(30);

bool found = list.Contains(20);
Console.WriteLine(found);

medium

A. 20

B. True

C. False

D. Compilation Error

Solution

1. Step 1: Understand LinkedList operations

   Elements 10, 20, 30 are added to the linked list. The Contains method checks if 20 exists in the list.

2. Step 2: Evaluate the Contains method result

   Since 20 was added, Contains(20) returns true. Console.WriteLine prints True.

3. Final Answer:

   True -> Option B

4. Quick Check:

   list.Contains(20) = True [OK]

Hint: LinkedList.Contains returns true if element exists [OK]

Common Mistakes:

• Assuming Contains returns the element instead of bool
• Confusing AddLast with AddFirst
• Expecting output 20 instead of True

4. Identify the error in this C# code snippet that tries to find a value in an array:

int[] arr = {1, 2, 3, 4};
if(arr.Contains(5))
{
    Console.WriteLine("Found");
}

medium

A. Contains method returns index, not bool

B. Array does not have Contains method

C. Contains method requires using System.Linq namespace

D. Array must be converted to List before Contains

Solution

1. Step 1: Check if Array.Contains is available by default

   Array itself does not have Contains method unless System.Linq is used which adds extension methods.

2. Step 2: Identify missing namespace

   Without 'using System.Linq;', Contains method call causes a compile error. Adding the namespace fixes it.

3. Final Answer:

   Contains method requires using System.Linq namespace -> Option C

4. Quick Check:

   Missing System.Linq causes Contains error [OK]

Hint: Add 'using System.Linq;' to use Contains on arrays [OK]

Common Mistakes:

• Thinking Array has Contains by default
• Confusing Contains return type
• Trying to convert array unnecessarily

5. You need to store a large collection of unique user IDs and quickly check if a user ID exists. Which data structure is best and why?

Choose the best option:

hard

A. Hash Map (Dictionary), because it provides O(1) average lookup time for existence checks

B. Linked List, because it allows easy insertion and deletion

C. Stack, because it supports fast push and pop operations

D. Array, because it uses less memory and supports fast index lookup

Solution

1. Step 1: Analyze requirements for storing unique IDs and fast existence check

   We need to quickly check if a user ID exists, so lookup speed is critical. Also, uniqueness is important.

2. Step 2: Compare data structures for lookup efficiency and uniqueness

   Array requires linear search for existence, O(n). Linked List also O(n). Stack is LIFO, not suitable for lookup. Hash Map (Dictionary) offers O(1) average lookup and can enforce uniqueness by keys.

3. Final Answer:

   Hash Map (Dictionary), because it provides O(1) average lookup time for existence checks -> Option A

4. Quick Check:

   Fast unique lookup = Hash Map [OK]

Hint: Use Hash Map for fast unique key existence checks [OK]

Common Mistakes:

• Choosing array for fast lookup without key index
• Confusing insertion speed with lookup speed
• Selecting stack which is not for lookup