Hash Map vs Array vs Linked List for Lookup in DSA C# - Compare & Choose

Challenge - 5 Problems

❓ Predict Output

intermediate

Lookup Time Comparison: Array vs Linked List

What is the output of the following C# code that searches for the number 5 in an array and a linked list?

DSA C#

using System;
using System.Collections.Generic;

class Program {
    static void Main() {
        int[] arr = {1, 3, 5, 7, 9};
        LinkedList<int> list = new LinkedList<int>(new int[] {1, 3, 5, 7, 9});

        bool foundInArray = false;
        foreach (int num in arr) {
            if (num == 5) {
                foundInArray = true;
                break;
            }
        }

        bool foundInList = false;
        foreach (int num in list) {
            if (num == 5) {
                foundInList = true;
                break;
            }
        }

        Console.WriteLine($"Found in array: {foundInArray}");
        Console.WriteLine($"Found in linked list: {foundInList}");
    }
}

A

Found in array: False
Found in linked list: True

B

Found in array: True
Found in linked list: True

C

Found in array: True
Found in linked list: False

D

Found in array: False
Found in linked list: False

💡 Hint

Both array and linked list contain the number 5, so searching should find it.

The code searches for the number 5 in both the array and the linked list. Since both contain 5, the output shows True for both.

❓ Predict Output

intermediate

Hash Map Lookup Result

What will be printed after running this C# code that uses a Dictionary to lookup a key?

DSA C#

using System;
using System.Collections.Generic;

class Program {
    static void Main() {
        Dictionary<string, int> map = new Dictionary<string, int>() {
            {"apple", 1},
            {"banana", 2},
            {"cherry", 3}
        };

        if (map.TryGetValue("banana", out int value)) {
            Console.WriteLine(value);
        } else {
            Console.WriteLine("Not found");
        }
    }
}

A 2

B Not found

C 3

D 1

💡 Hint

Check the value associated with the key "banana" in the dictionary.

The dictionary contains "banana" with value 2, so TryGetValue returns true and prints 2.

🧠 Conceptual

advanced

Time Complexity of Lookup Operations

Which data structure provides the fastest average lookup time for searching an element by key?

A Array with linear search

B Linked list with linear search

C Hash map with average constant time lookup

D Binary tree with linear search

💡 Hint

Consider average time complexity for each data structure's lookup.

Hash maps provide average O(1) lookup time, which is faster than linear search in arrays or linked lists.

❓ Predict Output

advanced

Effect of Collisions in Hash Map Lookup

What will be the output of this C# code that inserts keys causing collisions in a Dictionary and then looks up a key?

DSA C#

using System;
using System.Collections.Generic;

class Program {
    class BadHash {
        public int Value;
        public BadHash(int v) { Value = v; }
        public override int GetHashCode() { return 1; } // Forces collision
        public override bool Equals(object obj) {
            if (obj is BadHash other) return Value == other.Value;
            return false;
        }
    }

    static void Main() {
        Dictionary<BadHash, string> map = new Dictionary<BadHash, string>();
        map[new BadHash(1)] = "One";
        map[new BadHash(2)] = "Two";
        map[new BadHash(3)] = "Three";

        Console.WriteLine(map[new BadHash(2)]);
    }
}

A Two

B One

C Three

D KeyNotFoundException

💡 Hint

Even with collisions, dictionary uses Equals to find the correct key.

Despite all keys having the same hash code, the dictionary uses Equals to distinguish keys, so lookup for key with Value=2 returns "Two".

🧠 Conceptual

expert

Choosing Data Structure for Frequent Lookups and Insertions

You need a data structure that supports fast lookups and insertions by key with minimal memory overhead. Which is the best choice?

A Linked list

B Array

C Binary search tree

D Hash map

💡 Hint

Consider average time complexity and memory usage for each structure.

Hash maps provide average O(1) lookup and insertion, making them ideal for frequent operations with reasonable memory use.

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