Collision Handling Using Chaining in DSA C# - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is collision handling in hashing?

Collision handling is a method to resolve situations when two keys hash to the same index in a hash table.

beginner

Explain chaining in collision handling.

Chaining stores all elements that hash to the same index in a linked list at that index, allowing multiple items to be stored in one bucket.

beginner

What data structure is commonly used in chaining?

A linked list is commonly used to store multiple elements at the same hash index in chaining.

intermediate

How does chaining affect search time in a hash table?

Search time may increase if many elements collide and form a long linked list, but on average it remains efficient if the load factor is low.

intermediate

What is the main advantage of chaining over open addressing?

Chaining can handle an unlimited number of collisions by growing linked lists, while open addressing has limited slots and may require rehashing.

In chaining, what happens when two keys hash to the same index?

A The second key is discarded

B One key replaces the other

C The hash table resizes immediately

D Both keys are stored in a linked list at that index

Chaining stores all colliding keys in a linked list at the same index.

Which data structure is used in chaining to handle collisions?

A Array

B Queue

C Linked list

D Stack

Linked lists are used to store multiple elements at the same hash index in chaining.

What is a potential downside of chaining?

A It can cause long linked lists slowing down search

B It cannot handle collisions

C It wastes memory by storing empty slots

D It requires rehashing on every insert

Long linked lists from many collisions can slow down search operations.

How does chaining handle unlimited collisions?

A By storing colliding elements in linked lists

B By resizing the hash table

C By discarding extra elements

D By using open addressing

Chaining uses linked lists to store all colliding elements at the same index.

What is the load factor in a hash table?

A Number of collisions

B Number of keys divided by number of buckets

C Size of linked list

D Hash function complexity

Load factor is the ratio of keys stored to the number of buckets in the hash table.

Describe how chaining resolves collisions in a hash table.

Think about what happens when two friends want to sit on the same chair.

Explain the advantages and disadvantages of using chaining for collision handling.

Consider both how chaining helps and what problems it might cause.

Practice

1. What is the main idea behind collision handling using chaining in hash tables?

easy

A. Replace the existing element with the new one

B. Resize the hash table immediately when collision occurs

C. Store collided elements in a linked list at the same index

D. Ignore the new element causing collision

Solution

1. Step 1: Understand collision in hash tables

   When two keys hash to the same index, a collision happens.

2. Step 2: How chaining handles collisions

   Chaining stores all collided elements in a linked list at that index.

3. Final Answer:

   Store collided elements in a linked list at the same index -> Option C

4. Quick Check:

   Chaining = linked list at index [OK]

Hint: Chaining means linked list at one index [OK]

Common Mistakes:

• Thinking chaining replaces old elements
• Confusing chaining with open addressing
• Assuming collisions are ignored

2. Which of the following is the correct way to declare a hash table with chaining in C# using List<T> for chaining?

easy

A. Dictionary hashTable = new Dictionary();

B. Dictionary hashTable = new Dictionary();

C. List hashTable = new List();

D. Dictionary hashTable = new Dictionary();

Solution

1. Step 1: Understand chaining data structure

   Chaining uses a list to store multiple values per key index.

2. Step 2: Correct C# syntax for dictionary with list values

   Dictionary means keys are int, values are lists of strings.

3. Final Answer:

   Dictionary hashTable = new Dictionary(); -> Option D

4. Quick Check:

   Dictionary with List values = Dictionary hashTable = new Dictionary(); [OK]

Hint: Use Dictionary with List as value for chaining [OK]

Common Mistakes:

• Using List with two generic types (invalid)
• Using Dictionary with simple string values only
• Confusing key and value types

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

var hashTable = new Dictionary();
int key = 2;
if (!hashTable.ContainsKey(key))
    hashTable[key] = new List<string>();
hashTable[key].Add("apple");
hashTable[key].Add("banana");

foreach(var item in hashTable[key])
    Console.Write(item + " ");

medium

A. No output

B. apple banana

C. apple

D. banana apple

Solution

1. Step 1: Initialize dictionary and add list for key 2

   Since key 2 is not present, a new list is created and assigned.

2. Step 2: Add "apple" and "banana" to list at key 2

   Both strings are added in order: "apple", then "banana".

3. Step 3: Print all items in list at key 2

   Loop prints "apple " then "banana " in order.

4. Final Answer:

   apple banana -> Option B

5. Quick Check:

   List order preserved = apple banana [OK]

Hint: Items print in insertion order in chaining list [OK]

Common Mistakes:

• Assuming reversed order output
• Forgetting to initialize list before adding
• Expecting no output if key missing

4. Identify the error in this C# code snippet implementing chaining:

var hashTable = new Dictionary();
int key = 5;
hashTable[key].Add("orange");

medium

A. Key does not exist; list not initialized before Add

B. Dictionary cannot have List as value type

C. Add method does not exist for List

D. Key must be string, not int

Solution

1. Step 1: Check if key 5 exists in dictionary

   Key 5 is not added yet, so hashTable[key] is null.

2. Step 2: Adding to null list causes runtime error

   Cannot call Add on null; list must be initialized first.

3. Final Answer:

   Key does not exist; list not initialized before Add -> Option A

4. Quick Check:

   Initialize list before Add [OK]

Hint: Always initialize list before adding elements [OK]

Common Mistakes:

• Assuming dictionary auto-creates lists
• Confusing key types
• Thinking Add method is invalid

5. You want to implement a hash table with chaining in C# that stores multiple values per key and supports removing a specific value from the chain. Which approach correctly supports this feature?

hard

A. Use Dictionary and call Remove(value) on the list at the key

B. Use Dictionary and overwrite value to remove

C. Use Dictionary but clear entire list to remove one value

D. Use Dictionary but do not implement removal

Solution

1. Step 1: Understand chaining with multiple values per key

   Chaining uses a list to hold multiple values at one key.

2. Step 2: Removing a specific value requires list's Remove method

   List<string> supports Remove(value) to delete one item without affecting others.

3. Step 3: Other options either overwrite or clear all values, not specific removal

   Overwriting loses all values; clearing removes all; ignoring removal is incomplete.

4. Final Answer:

   Use Dictionary and call Remove(value) on the list at the key -> Option A

5. Quick Check:

   List Remove supports specific value deletion [OK]

Hint: Use List and Remove(value) to delete specific chained item [OK]

Common Mistakes:

• Overwriting value instead of removing one
• Clearing entire list to remove one item
• Ignoring removal feature