Linked List vs Array When to Choose Which in DSA C++ - Compare & Choose

Challenge - 5 Problems

🧠 Conceptual

intermediate

Choosing Between Linked List and Array for Frequent Insertions

You need to store a collection of items where insertions happen frequently at the beginning and middle of the collection. Which data structure is generally better to use?

A Linked List, because it allows efficient insertions without shifting elements.

B Linked List, because it provides faster access to elements by index.

C Array, because it allows fast random access to elements.

D Array, because it uses less memory than linked lists.

💡 Hint

Think about what happens when you insert elements in the middle of an array versus a linked list.

Linked lists allow insertions and deletions at any position efficiently because they only change pointers, while arrays require shifting elements which is costly.

❓ Predict Output

intermediate

Output of Array vs Linked List Access Time Simulation

Consider the following C++ code simulating access times for array and linked list. What is the printed output?

DSA C++

int arr[3] = {10, 20, 30};
struct Node {
  int val;
  Node* next;
};
Node n3 = {30, nullptr};
Node n2 = {20, &n3};
Node n1 = {10, &n2};

std::cout << arr[1] << " " << n1.next->val << std::endl;

A 20 30

B 10 30

C 10 20

D 20 20

💡 Hint

Check the index used for array and the pointer navigation in linked list.

arr[1] accesses the second element which is 20. n1.next points to n2 which has value 20.

❓ Predict Output

advanced

Resulting List After Mixed Insertions in Linked List

What is the state of the linked list after the following operations?

DSA C++

struct Node {
  int val;
  Node* next;
};

Node* head = new Node{1, nullptr};
head->next = new Node{3, nullptr};
Node* newNode = new Node{2, nullptr};
newNode->next = head->next;
head->next = newNode;

// Print list
Node* curr = head;
while(curr) {
  std::cout << curr->val << " -> ";
  curr = curr->next;
}
std::cout << "null" << std::endl;

A 2 -> 1 -> 3 -> null

B 1 -> 2 -> 3 -> null

C 3 -> 2 -> 1 -> null

D 1 -> 3 -> 2 -> null

💡 Hint

Trace the pointer changes step by step.

The new node with value 2 is inserted between 1 and 3, so the list becomes 1 -> 2 -> 3 -> null.

🧠 Conceptual

advanced

Memory Usage Comparison Between Array and Linked List

Which statement correctly describes the memory usage difference between arrays and linked lists?

A Arrays and linked lists use the same amount of memory for the same number of elements.

B Arrays use more memory because they store extra pointers for each element.

C Linked lists use more memory because each node stores data and a pointer to the next node.

D Linked lists use less memory because they allocate memory only when needed.

💡 Hint

Consider what extra information each linked list node stores compared to array elements.

Each linked list node stores the data plus a pointer to the next node, increasing memory usage compared to arrays which store only data.

🚀 Application

expert

Choosing Data Structure for Real-Time Queue with Frequent Deletions

You are designing a real-time system that requires a queue with frequent deletions from the front and occasional insertions at the back. Which data structure is most suitable and why?

A Linked List, because it allows efficient deletions from the front without shifting elements.

B Array, because it allows fast random access and easy insertions at the back.

C Array, because it uses less memory and supports dynamic resizing automatically.

D Linked List, because it provides faster access to elements by index.

💡 Hint

Think about what happens when deleting elements from the front in arrays versus linked lists.

Linked lists allow efficient deletions from the front by moving the head pointer, while arrays require shifting all elements which is costly.

Practice

1. Which data structure is better when you need fast random access to elements by index?

easy

A. Queue

B. Array

C. Stack

D. Linked List

Solution

1. Step 1: Understand random access requirement

   Random access means accessing any element directly by its position quickly.

2. Step 2: Compare array and linked list access times

   Arrays allow direct access by index in constant time, while linked lists require traversal.

3. Final Answer:

   Array -> Option B

4. Quick Check:

   Random access = Array [OK]

Hint: For fast index access, choose arrays [OK]

Common Mistakes:

• Thinking linked lists provide fast random access
• Confusing stack or queue with random access
• Ignoring traversal cost in linked lists

2. Which of the following is the correct way to declare a dynamic array in C++?

easy

A. int arr[] = new int(10);

B. int arr[10];

C. int arr = new int[10];

D. int* arr = new int[10];

Solution

1. Step 1: Identify dynamic array syntax in C++

   Dynamic arrays use pointers with 'new' keyword and size in brackets.

2. Step 2: Check each option

   int* arr = new int[10]; correctly uses 'int* arr = new int[10];'. Others are either static arrays or incorrect syntax.

3. Final Answer:

   int* arr = new int[10]; -> Option D

4. Quick Check:

   Dynamic array = new int[size] [OK]

Hint: Use 'new' with pointer for dynamic arrays in C++ [OK]

Common Mistakes:

• Using static array syntax for dynamic arrays
• Missing pointer (*) in declaration
• Incorrect use of parentheses instead of brackets

3. What will be the output of the following C++ code snippet?

struct Node {
  int data;
  Node* next;
};

Node* head = new Node{1, nullptr};
head->next = new Node{2, nullptr};
head->next->next = new Node{3, nullptr};

Node* temp = head;
while(temp != nullptr) {
  std::cout << temp->data << " ";
  temp = temp->next;
}

medium

A. 1 2 3

B. 3 2 1

C. 1 3 2

D. Compilation error

Solution

1. Step 1: Understand linked list creation

   Nodes are created with values 1, 2, 3 linked in order: head -> 1 -> 2 -> 3 -> nullptr.

2. Step 2: Trace the while loop output

   Loop prints data of each node from head to end: 1, then 2, then 3, each followed by space.

3. Final Answer:

   1 2 3 -> Option A

4. Quick Check:

   Traverse linked list prints 1 2 3 [OK]

Hint: Traverse linked list from head to print data [OK]

Common Mistakes:

• Confusing order of nodes
• Forgetting to update temp pointer
• Assuming array-like indexing

4. Identify the error in the following code that tries to insert a new node at the beginning of a linked list:

struct Node {
  int data;
  Node* next;
};

void insertAtHead(Node* head, int val) {
  Node* newNode = new Node{val, nullptr};
  newNode->next = head;
  head = newNode;
}

medium

A. head pointer is passed by value, so original list is not updated

B. Missing return type in function

C. newNode->next should be nullptr

D. Memory leak due to not deleting old head

Solution

1. Step 1: Analyze function parameter passing

   head is passed by value, so changes inside function do not affect original head pointer.

2. Step 2: Understand effect on linked list

   Assigning head = newNode only changes local copy; original list head remains unchanged.

3. Final Answer:

   head pointer is passed by value, so original list is not updated -> Option A

4. Quick Check:

   Pass pointer by reference to update list head [OK]

Hint: Pass head by reference to modify original list [OK]

Common Mistakes:

• Assuming pointer changes affect caller without reference
• Incorrect next pointer assignment
• Ignoring memory management

5. You need to store a large collection of elements where insertions and deletions happen frequently at random positions. Which data structure is more suitable and why?

hard

A. Array, because it provides fast random access

B. Array, because it uses less memory than linked list

C. Linked List, because it allows efficient insertions and deletions without shifting elements

D. Linked List, because it supports binary search efficiently

Solution

1. Step 1: Understand operation requirements

   Frequent insertions and deletions at random positions require efficient element rearrangement.

2. Step 2: Compare array and linked list for these operations

   Arrays require shifting elements on insert/delete, which is costly. Linked lists adjust pointers, making these operations efficient.

3. Final Answer:

   Linked List, because it allows efficient insertions and deletions without shifting elements -> Option C

4. Quick Check:

   Frequent insert/delete = Linked List [OK]

Hint: Use linked list for frequent insertions/deletions [OK]

Common Mistakes:

• Choosing array for frequent insert/delete ignoring shift cost
• Thinking linked list supports fast random access
• Confusing binary search applicability