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

Challenge - 5 Problems

❓ Predict Output

intermediate

Lookup Time Complexity Comparison

Given the following lookup operations on different data structures, what is the output showing the time complexity for each lookup?

DSA C

/* Lookup time complexities */
#include <stdio.h>

int main() {
    printf("Array lookup: O(1)\n");
    printf("Linked List lookup: O(n)\n");
    printf("Hash Map lookup: O(1) average, O(n) worst\n");
    return 0;
}

A Array lookup: O(1)\nLinked List lookup: O(1)\nHash Map lookup: O(n)

B Array lookup: O(n)\nLinked List lookup: O(1)\nHash Map lookup: O(1)

C Array lookup: O(1)\nLinked List lookup: O(n)\nHash Map lookup: O(1) average, O(n) worst

D Array lookup: O(n)\nLinked List lookup: O(n)\nHash Map lookup: O(n)

💡 Hint

Think about how each data structure accesses elements: arrays by index, linked lists by traversal, hash maps by hashing keys.

Arrays allow direct access by index, so lookup is O(1). Linked lists require traversal, so lookup is O(n). Hash maps use hashing for average O(1) lookup but can degrade to O(n) if many collisions occur.

🧠 Conceptual

intermediate

Best Data Structure for Frequent Lookups

Which data structure is generally best for frequent fast lookups by key?

A Hash Map

B Array

C Stack

D Linked List

💡 Hint

Consider which structure uses hashing to find keys quickly.

Hash maps provide average O(1) lookup time by using a hash function to find keys quickly. Arrays require index or linear search, and linked lists require traversal.

🔧 Debug

advanced

Why is Linked List Lookup Slow?

Consider a linked list with 1000 elements. Why does lookup by value take longer compared to an array?

A Because linked lists use hashing which is slower than arrays.

B Because linked lists store elements contiguously in memory.

C Because linked lists have fixed size and arrays can grow dynamically.

D Because linked lists store elements non-contiguously and require traversal from head to find a value.

💡 Hint

Think about how you find an element in a linked list versus an array.

Linked lists store elements scattered in memory and you must check each node from the start until you find the value, making lookup O(n). Arrays store elements contiguously and allow direct index access.

❓ Predict Output

advanced

Hash Map Collision Impact on Lookup

What is the output of this code simulating hash map lookup with collisions?

DSA C

#include <stdio.h>
#include <string.h>

int hash(char *key) {
    return strlen(key) % 3; // simple hash function
}

int main() {
    char *keys[] = {"cat", "dog", "bear", "eagle"};
    int buckets[3] = {0};
    for (int i = 0; i < 4; i++) {
        int h = hash(keys[i]);
        buckets[h]++;
    }
    printf("Bucket counts: %d %d %d\n", buckets[0], buckets[1], buckets[2]);
    return 0;
}

A Bucket counts: 2 1 1

B Bucket counts: 1 2 1

C Bucket counts: 1 1 2

D Bucket counts: 0 2 2

💡 Hint

Calculate hash for each key using strlen(key) % 3 and count how many keys fall into each bucket.

Keys and lengths: cat(3)%3=0, dog(3)%3=0, bear(4)%3=1, eagle(5)%3=2. Bucket 0 gets cat and dog (collision), bucket 1 gets bear, bucket 2 gets eagle. Output: Bucket counts: 2 1 1. In a real hash map, collisions in a bucket would require linear search within the bucket, impacting lookup time.

🧠 Conceptual

expert

Choosing Data Structure for Mixed Operations

You need a data structure that supports fast lookup, insertion, and ordered traversal. Which is the best choice?

A Array

B Hash Map with Doubly Linked List

C Hash Map

D Linked List

💡 Hint

Think about combining fast lookup with maintaining order.

Hash maps provide fast lookup and insertion but do not maintain order. Linked lists maintain order but have slow lookup. Combining a hash map with a doubly linked list allows fast lookup and ordered traversal.