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

Concept Flow - Hash Map vs Array vs Linked List for Lookup

Start Lookup

↓

Choose Data Structure

↓

Check Index

↓

Return Value

↓

End Lookup

Lookup starts by choosing the data structure, then uses index for array, node traversal for linked list, or hash computation for hash map to find the value.

Execution Sample

DSA C#

int[] arr = {10, 20, 30};
int lookupIndex = 1;
int value = arr[lookupIndex];

// Linked List lookup
// Hash Map lookup

This code looks up a value at index 1 in an array; similar logic applies for linked list traversal and hash map key lookup.

Execution Table

Step	Data Structure	Operation	Details	Result	Visual State
1	Array	Lookup by index	Index = 1	Value = 20	[10, 20, 30]
2	Linked List	Traverse nodes	Start at head (10)	Not found, move next	10 -> 20 -> 30 -> null
3	Linked List	Traverse nodes	At node with value 20	Found value 20	10 -> 20 -> 30 -> null
4	Hash Map	Compute hash	Key = 1, hash = 1	Go to bucket 1	{0:10, 1:20, 2:30}
5	Hash Map	Check bucket	Bucket 1 has key 1	Found value 20	{0:10, 1:20, 2:30}
6	End	Lookup complete	Value found	Value = 20	No change

💡 Lookup ends after value is found or all nodes checked.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	Final
lookupIndex	1	1	1	1	1	1	1
Array Value	N/A	20	20	20	20	20	20
LinkedList Current Node	Head (10)	10	20	20	20	20	20
Hash Map Bucket	N/A	N/A	N/A	N/A	Bucket 1	Bucket 1	Bucket 1
Hash Map Value	N/A	N/A	N/A	N/A	N/A	20	20

Key Moments - 3 Insights

Why does array lookup use direct index but linked list requires traversal?

Array elements are stored contiguously, so accessing by index is direct (see Step 1). Linked list nodes are linked by pointers, so we must check each node one by one (see Steps 2 and 3).

How does hash map find the value quickly compared to linked list?

Hash map uses a hash function to jump directly to the bucket where the key should be (Step 4), avoiding traversal of all elements unlike linked list (Steps 2 and 3).

What happens if the key is not found in the linked list or hash map?

In linked list, traversal continues until the end (null) is reached without finding the key. In hash map, if the bucket is empty or key not present, lookup fails after checking the bucket.

Visual Quiz - 3 Questions

Look at the execution table, what is the value found at Step 3 in the linked list?

A 20

B 10

C 30

D null

💡 Hint

Check the 'Result' column at Step 3 in the execution table.

At which step does the hash map compute the hash for the key?

A Step 3

B Step 4

C Step 2

D Step 5

💡 Hint

Look for 'Compute hash' operation in the execution table.

If the array index was 2 instead of 1, what would be the value at Step 1?

A 10

B 20

C 30

D null

💡 Hint

Refer to the array values and index used in Step 1.

Concept Snapshot

Lookup in Array: Direct access by index, very fast O(1).
Lookup in Linked List: Must check nodes one by one, slower O(n).
Lookup in Hash Map: Uses hash function to jump to bucket, average O(1).
Arrays store elements contiguously; linked lists use nodes with pointers.
Hash maps handle collisions by buckets or chaining.
Choose data structure based on lookup speed and data size.

Full Transcript

This visual execution compares lookup operations in array, linked list, and hash map. Array lookup uses direct index access to get the value quickly. Linked list lookup requires traversing nodes one by one until the value is found. Hash map lookup computes a hash to jump directly to the bucket containing the key, then retrieves the value. The execution table shows each step with the data structure state and results. Variable tracker follows key variables like current node and bucket. Key moments clarify why arrays are fast for lookup, why linked lists need traversal, and how hash maps speed up search using hashing. The quiz tests understanding of these steps and results. This helps beginners see how lookup differs in these structures visually and step-by-step.

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