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.

Practice

1. Which data structure provides the fastest average lookup time for finding an element by key?

easy

A. Stack

B. Hash Map

C. Linked List

D. Array

Solution

1. Step 1: Understand lookup times for each data structure

   Hash Map uses hashing to find keys quickly, usually in constant time O(1). Arrays require scanning or index access, O(1) if index known but O(n) if searching by value. Linked Lists require scanning all nodes, O(n).

2. Step 2: Compare average lookup times

   Hash Map is fastest on average for key lookup due to direct hashing. Arrays and Linked Lists are slower for key-based search.

3. Final Answer:

   Hash Map -> Option B

4. Quick Check:

   Fastest lookup by key = Hash Map [OK]

Hint: Hash Map lookup is usually O(1), faster than arrays or lists [OK]

Common Mistakes:

• Confusing array index access with key lookup
• Thinking linked list lookup is fast
• Assuming stack is good for key lookup

2. Which of the following is the correct way to declare an array of 5 integers in C?

easy

A. int arr(5);

B. int arr = new int[5];

C. array<int> arr(5);

D. int arr[5];

Solution

1. Step 1: Recall C array declaration syntax

   In C, arrays are declared with type, name, and size in square brackets, e.g., int arr[5];

2. Step 2: Check each option

   int arr[5]; matches correct C syntax. int arr = new int[5]; uses C++ style new keyword. int arr(5); is invalid syntax. array<int> arr(5); is C++ template syntax, not C.

3. Final Answer:

   int arr[5]; -> Option D

4. Quick Check:

   C array declaration = int arr[5]; [OK]

Hint: C arrays use square brackets for size, no new keyword [OK]

Common Mistakes:

• Using C++ new keyword in C
• Using parentheses instead of brackets
• Confusing C++ templates with C arrays

3. Consider the following C code snippet using a linked list for lookup:

typedef struct Node {
  int key;
  int value;
  struct Node* next;
} Node;

int lookup(Node* head, int key) {
  Node* current = head;
  while (current != NULL) {
    if (current->key == key) {
      return current->value;
    }
    current = current->next;
  }
  return -1;
}

// List: (1,10) -> (2,20) -> (3,30) -> NULL
// Call: lookup(head, 2);

What is the output of lookup(head, 2);?

medium

A. 20

B. 10

C. 30

D. -1

Solution

1. Step 1: Trace the linked list nodes

   The list nodes have keys and values: (1,10), (2,20), (3,30). The function searches for key 2.

2. Step 2: Follow lookup function logic

   Starting at head (key=1), key != 2, move next. Next node key=2 matches, return value 20.

3. Final Answer:

   20 -> Option A

4. Quick Check:

   Found key 2 with value 20 [OK]

Hint: Linked list lookup scans nodes until key found [OK]

Common Mistakes:

• Returning value of first node always
• Confusing key and value fields
• Returning -1 when key exists

4. The following C code attempts to find a value in an array but has a bug:

int find(int arr[], int size, int target) {
  for (int i = 0; i <= size; i++) {
    if (arr[i] == target) {
      return i;
    }
  }
  return -1;
}

What is the bug in this code?

medium

A. Return value should be arr[i], not i

B. Function should return void

C. Loop condition should be i < size, not i <= size

D. Array parameter should be int* arr

Solution

1. Step 1: Analyze loop boundary

   The loop runs from i = 0 to i <= size, which means i can be equal to size. Array indices go from 0 to size-1, so arr[size] is out of bounds.

2. Step 2: Correct loop condition

   Change condition to i < size to avoid accessing out of bounds memory.

3. Final Answer:

   Loop condition should be i < size, not i <= size -> Option C

4. Quick Check:

   Array index must be less than size [OK]

Hint: Array indices go 0 to size-1, use i < size in loops [OK]

Common Mistakes:

• Using <= size causes out-of-bounds access
• Returning arr[i] instead of index
• Changing return type unnecessarily

5. You need to store a large number of unique integer keys with associated values and perform frequent lookups. Which data structure is best and why?

hard

A. Hash Map - because it provides average constant time lookup

B. Linked List - because it allows easy insertion

C. Array - because it uses contiguous memory

D. Binary Search Tree - because it keeps keys sorted

Solution

1. Step 1: Understand requirements

   We need to store many unique keys with values and do frequent lookups efficiently.

2. Step 2: Evaluate data structures

   Array is inefficient for key lookup unless keys are indices. Linked List has slow O(n) lookup. Binary Search Tree has O(log n) lookup but more complex. Hash Map offers average O(1) lookup, ideal for fast access.

3. Final Answer:

   Hash Map - because it provides average constant time lookup -> Option A

4. Quick Check:

   Fast lookup with many keys = Hash Map [OK]

Hint: For fast key lookup, choose Hash Map over arrays or lists [OK]

Common Mistakes:

• Choosing array for non-index keys
• Ignoring lookup speed in linked lists
• Preferring BST without sorting need