HashMap Implementation from Scratch in DSA Rust - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a HashMap?

A HashMap is a data structure that stores key-value pairs. It uses a hash function to convert keys into an index where values are stored, allowing fast data retrieval.

beginner

Why do we use a hash function in a HashMap?

A hash function converts a key into a number (index) to quickly find where the value is stored in the underlying array. It helps in fast lookup and insertion.

intermediate

What is collision in a HashMap and how can it be handled?

Collision happens when two keys produce the same index. It can be handled by methods like chaining (storing multiple items in a list at the same index) or open addressing (finding another empty slot).

intermediate

In Rust, what is a simple way to store multiple values at the same index to handle collisions?

We can use a vector (Vec) of key-value pairs at each index to store multiple entries. This is called chaining.

beginner

What are the main operations of a HashMap?

The main operations are: insert (add a key-value pair), get (retrieve value by key), and remove (delete a key-value pair).

What does a hash function do in a HashMap?

A Removes duplicate keys

B Sorts the keys alphabetically

C Stores values directly

D Converts a key into an index

A hash function converts a key into an index to find where the value is stored.

Which method can handle collisions by storing multiple key-value pairs at the same index?

A Chaining

B Binary search

C Open addressing

D Sorting

Chaining stores multiple key-value pairs in a list at the same index to handle collisions.

What is the time complexity of average lookup in a well-implemented HashMap?

A O(1)

B O(log n)

C O(n)

D O(n log n)

Average lookup time in a HashMap is O(1) due to direct indexing via the hash function.

In Rust, which data structure is commonly used to store multiple key-value pairs at one index for collision handling?

A HashSet

B Vec (vector)

C Option

D Tuple

A Vec (vector) is used to store multiple key-value pairs at one index for chaining.

Which operation is NOT a main operation of a HashMap?

A Insert

B Get

C Sort

D Remove

Sorting is not a main operation of a HashMap.

Explain how a HashMap stores and retrieves data using a hash function and handles collisions.

Think about how keys become positions and what happens if two keys want the same spot.

Describe how you would implement a simple HashMap in Rust from scratch, focusing on data storage and collision handling.

Imagine an array where each slot can hold multiple pairs to avoid losing data.

Practice

1. What is the main purpose of a HashMap in data structures?

easy

A. To store elements in a queue-like manner

B. To store elements in sorted order

C. To store key-value pairs for fast lookup

D. To store elements in a stack-like manner

Solution

1. Step 1: Understand HashMap functionality

   A HashMap stores data as key-value pairs, allowing quick access by key.

2. Step 2: Compare with other data structures

   Unlike stacks or queues, HashMaps do not maintain order but focus on fast lookup.

3. Final Answer:

   To store key-value pairs for fast lookup -> Option C

4. Quick Check:

   HashMap = key-value fast access [OK]

Hint: HashMap = fast key-value storage [OK]

Common Mistakes:

• Confusing HashMap with sorted structures
• Thinking HashMap stores elements in order
• Mixing HashMap with stack or queue behavior

2. Which of the following is the correct Rust syntax to create an empty HashMap from scratch?

easy

A. let map = HashMap::empty();

B. let map = new HashMap();

C. let map = HashMap();

D. let map = HashMap::new();

Solution

1. Step 1: Recall Rust HashMap creation syntax

   Rust uses HashMap::new() to create an empty HashMap.

2. Step 2: Check other options for Rust correctness

   Options B, C, and D are not valid Rust syntax for HashMap creation.

3. Final Answer:

   let map = HashMap::new(); -> Option D

4. Quick Check:

   Rust HashMap empty = HashMap::new() [OK]

Hint: Use HashMap::new() to create empty map [OK]

Common Mistakes:

• Using constructor syntax like new HashMap()
• Calling HashMap() without ::new()
• Using non-existent methods like empty()

3. Consider this simplified Rust code snippet for a custom HashMap insert method:

let mut map = MyHashMap::new();
map.insert("apple", 3);
map.insert("banana", 5);
map.insert("apple", 7);
println!("{:?}", map.get("apple"));

What will be printed?

medium

A. Some(3)

B. Some(7)

C. None

D. Some(5)

Solution

1. Step 1: Analyze insertions for key "apple"

   First insert sets "apple" to 3, second insert updates "apple" to 7.

2. Step 2: Check the get method output

   Getting "apple" returns the latest value, which is 7, wrapped in Some.

3. Final Answer:

   Some(7) -> Option B

4. Quick Check:

   Last insert value = get output [OK]

Hint: Last inserted value for key is returned [OK]

Common Mistakes:

• Assuming first inserted value remains
• Confusing None with Some value
• Mixing values of different keys

4. Given this Rust snippet for a custom HashMap insert method, what is the bug?

fn insert(&mut self, key: K, value: V) {
    let index = self.hash(&key) % self.buckets.len();
    if self.buckets[index].is_empty() {
        self.buckets[index].push((key, value));
    } else {
        self.buckets[index].push((key, value));
    }
}

medium

A. It does not update value if key exists, causing duplicates

B. It uses wrong hash function signature

C. It does not handle empty buckets correctly

D. It forgets to resize buckets when full

Solution

1. Step 1: Review insert logic for existing keys

   The code always pushes new (key, value) pairs without checking for existing keys.

2. Step 2: Identify consequence of missing update

   This causes duplicate keys in the same bucket, breaking expected HashMap behavior.

3. Final Answer:

   It does not update value if key exists, causing duplicates -> Option A

4. Quick Check:

   Insert must update existing key, not just push [OK]

Hint: Check for existing key before inserting [OK]

Common Mistakes:

• Ignoring key existence before insert
• Assuming push replaces existing entries
• Missing bucket resizing logic

5. You are implementing a custom HashMap in Rust with separate chaining. Which approach best handles collisions and updates existing keys efficiently?

hard

A. Store key-value pairs in a vector at each bucket; on insert, search vector for key to update or append

B. Use a single vector for all keys without hashing; append all pairs sequentially

C. Replace entire bucket vector on each insert without checking keys

D. Use a linked list but never check for existing keys before inserting

Solution

1. Step 1: Understand separate chaining collision handling

   Separate chaining uses a list (vector) at each bucket to store multiple key-value pairs.

2. Step 2: Efficient insert requires checking existing keys

   Before inserting, search the bucket's vector for the key to update value or append if not found.

3. Step 3: Evaluate options

   Store key-value pairs in a vector at each bucket; on insert, search vector for key to update or append correctly describes this approach; others ignore key checks or hashing.

4. Final Answer:

   Store key-value pairs in a vector at each bucket; on insert, search vector for key to update or append -> Option A

5. Quick Check:

   Separate chaining + key check = efficient insert [OK]

Hint: Search bucket vector for key before insert [OK]

Common Mistakes:

• Ignoring key existence in bucket
• Not using hashing for bucket selection
• Replacing bucket data blindly