Hash Map vs Array vs Linked List for Lookup in DSA Rust - Key Differences

Recall & Review

beginner

What is the average time complexity of looking up an element in a Hash Map?

The average time complexity for lookup in a Hash Map is O(1), meaning it can find an element very quickly regardless of the number of items.

beginner

How does lookup time in an Array compare to a Linked List?

In an Array, lookup by index is O(1) because elements are stored contiguously. In a Linked List, lookup is O(n) because you must check each node one by one.

beginner

Why is lookup in a Linked List slower than in an Array?

A Linked List stores elements in separate nodes connected by pointers, so to find an element, you must start at the head and move through nodes one by one, making it slower than direct access in an Array.

beginner

What is a real-life example to understand Hash Map lookup?

Think of a Hash Map like a library index card system where you use a book's title (key) to quickly find its location (value) without searching every book on the shelf.

beginner

Which data structure is best for fast lookup when you have a key and why?

A Hash Map is best for fast lookup by key because it uses a hash function to jump directly to the data location, avoiding the need to check each element.

What is the time complexity of looking up an element by index in an Array?

A O(n^2)

B O(n)

C O(1)

D O(log n)

Arrays allow direct access by index, so lookup is constant time O(1).

Which data structure requires checking nodes one by one to find an element?

A Linked List

B Hash Map

C Array

D Binary Search Tree

Linked Lists store elements in nodes connected by pointers, so you must traverse nodes sequentially.

Why is Hash Map lookup usually faster than Array lookup by value?

A Because Hash Maps store elements contiguously

B Because Arrays use pointers

C Because Arrays are unordered

D Because Hash Maps use a hash function to jump directly to the element

Hash Maps use a hash function to find the element location quickly without scanning all elements.

What happens if two keys hash to the same value in a Hash Map?

A The Hash Map uses a collision resolution method

B The Hash Map crashes

C The second key overwrites the first

D The keys are merged

Hash Maps handle collisions by methods like chaining or open addressing to store multiple keys with the same hash.

Which data structure is best when you need to maintain order and fast lookup by index?

A Hash Map

B Array

C Linked List

D Stack

Arrays maintain order and allow fast lookup by index with O(1) time.

Explain the differences in lookup speed between Hash Map, Array, and Linked List.

Think about how each data structure stores and accesses data.

Describe a real-life analogy to understand how a Hash Map works for lookup.

Imagine finding a book quickly in a large library.

Practice

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

easy

A. Array

B. Vector

C. Linked List

D. Hash Map

Solution

1. Step 1: Understand lookup times for data structures

   Hash Map uses hashing to find keys quickly, usually in constant time O(1).

2. Step 2: Compare with Array and Linked List

   Array and Linked List require searching elements linearly, which is slower (O(n)).

3. Final Answer:

   Hash Map -> Option D

4. Quick Check:

   Fastest lookup = Hash Map [OK]

Hint: Hash Map uses hashing for O(1) lookup, arrays and lists are slower [OK]

Common Mistakes:

• Thinking arrays have O(1) lookup for keys (only for indices)
• Confusing linked list traversal with direct access
• Assuming vector is same as hash map for lookup

2. Which Rust syntax correctly declares a mutable HashMap named map with String keys and i32 values?

easy

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

B. let mut map: HashMap = HashMap::new();

C. let mut map = HashMap::new();

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

Solution

1. Step 1: Check correct syntax for mutable HashMap declaration

   In Rust, let mut declares a mutable variable. The type is HashMap<String, i32>. The constructor is HashMap::new().

2. Step 2: Identify errors in other options

   let map = HashMap::new(); misses mut. let mut map = HashMap::new(); has wrong generic syntax. let map: HashMap = HashMap::new(); swaps key and value types.

3. Final Answer:

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

4. Quick Check:

   Mutable HashMap syntax = let mut map: HashMap = HashMap::new(); [OK]

Hint: Use let mut with HashMap::new() and correct generics [OK]

Common Mistakes:

• Forgetting mut keyword for mutable map
• Wrong generic syntax with new()
• Swapping key and value types

3. What is the output of this Rust code snippet?

use std::collections::HashMap;

fn main() {
    let mut map = HashMap::new();
    map.insert("apple", 3);
    map.insert("banana", 2);
    map.insert("orange", 5);

    let lookup = map.get("banana");
    println!("{:?}", lookup);
}

medium

A. None

B. 2

C. Some(2)

D. banana

Solution

1. Step 1: Understand HashMap get method return type

   The get method returns an Option type: Some(&value) if key exists, else None.

2. Step 2: Check if key "banana" exists and its value

   Key "banana" exists with value 2, so get("banana") returns Some(&2). Printing with {:?} shows Some(2).

3. Final Answer:

   Some(2) -> Option C

4. Quick Check:

   HashMap get returns Option = Some(2) [OK]

Hint: HashMap get returns Option; Some(value) if found, else None [OK]

Common Mistakes:

• Expecting direct value instead of Option
• Confusing None with Some
• Printing key instead of value

4. Identify the error in this Rust code that tries to find a value in a linked list:

struct Node {
    value: i32,
    next: Option,
}

fn find(head: &Option, target: i32) -> bool {
    let mut current = head;
    while current.is_some() {
        if current.as_ref().unwrap().value == target {
            return true;
        }
        current = current.unwrap().next;
    }
    false
}

medium

A. Using unwrap() twice causes ownership error

B. Missing mutable keyword for current

C. Incorrect comparison operator in if statement

D. next field should be a reference, not Box

Solution

1. Step 1: Analyze ownership in while loop

   Calling unwrap() on current moves ownership, so current cannot be used again.

2. Step 2: Identify why unwrap() causes error

   Using unwrap() twice consumes current, causing ownership error. Instead, use references or pattern matching.

3. Final Answer:

   Using unwrap() twice causes ownership error -> Option A

4. Quick Check:

   Ownership error due to unwrap() usage [OK]

Hint: unwrap() moves ownership; use references to avoid errors [OK]

Common Mistakes:

• Ignoring ownership rules with unwrap()
• Thinking mutable needed for current here
• Confusing Box with reference types

5. You need to store a large collection of user IDs (integers) and quickly check if a user ID exists. Which data structure is best and why? Choose the best option considering lookup speed and memory efficiency.

hard

A. Hash Map, because it offers average constant time lookup for keys

B. Array, because it provides constant time lookup for any integer key

C. Linked List, because it uses less memory and is easy to implement

D. Vector, because it automatically resizes and supports fast lookup

Solution

1. Step 1: Consider lookup speed for large collections

   Linked List has O(n) lookup, which is slow. Array lookup by index is O(1), but only if keys are indices, not arbitrary integers.

2. Step 2: Evaluate Hash Map and Vector

   Hash Map provides average O(1) lookup for arbitrary keys like user IDs. Vector lookup by index is O(1), but user IDs may not be continuous indices.

3. Final Answer:

   Hash Map, because it offers average constant time lookup for keys -> Option A

4. Quick Check:

   Best lookup for arbitrary keys = Hash Map [OK]

Hint: Use Hash Map for fast lookup of arbitrary keys like user IDs [OK]

Common Mistakes:

• Assuming array lookup works for arbitrary keys
• Choosing linked list for speed
• Confusing vector with hash map for key lookup