Linked List vs Array When to Choose Which in DSA C++ - Visual Comparison

Concept Flow - Linked List vs Array When to Choose Which

Start: Need to store elements

↓

Decide on access pattern

↓

Use chosen structure

↓

Perform operations efficiently

Shows decision steps to choose between linked list and array based on operation needs.

Execution Sample

DSA C++

int arr[5] = {1, 2, 3, 4, 5};
// Access arr[2]
// Insert 6 at end of linked list
struct Node {
    int val;
    Node* next;
    Node(int x) : val(x), next(nullptr) {}
};
Node* head = new Node(1);
head->next = new Node(2);
head->next->next = new Node(3);
// Insert 6 at end
Node* temp = head;
while(temp->next != nullptr) {
    temp = temp->next;
}
temp->next = new Node(6);

Shows array access by index and linked list insertion at end.

Execution Table

Step	Operation	Data Structure	Pointer Changes	Visual State
1	Initialize array with 5 elements	Array	N/A	[1, 2, 3, 4, 5]
2	Access element at index 2	Array	N/A	Accessed value: 3
3	Create linked list node with value 1	Linked List	head -> Node(1)	1 -> null
4	Add node with value 2	Linked List	head->next -> Node(2)	1 -> 2 -> null
5	Add node with value 3	Linked List	head->next->next -> Node(3)	1 -> 2 -> 3 -> null
6	Insert node with value 6 at end	Linked List	Node(3)->next -> Node(6)	1 -> 2 -> 3 -> 6 -> null
7	Access element at index 2 in array	Array	N/A	Value: 3
8	Traverse linked list to insert at end	Linked List	Traverse from head to Node(3)	Pointer at Node(3)
9	Link new node at end	Linked List	Node(3)->next = Node(6)	1 -> 2 -> 3 -> 6 -> null
10	End of operations	N/A	N/A	Final array: [1, 2, 3, 4, 5], Final list: 1 -> 2 -> 3 -> 6 -> null

💡 All operations completed; array remains fixed size; linked list updated with new node.

Variable Tracker

Variable	Start	After Step 3	After Step 5	After Step 6	Final
head	null	Node(1)	Node(1)	Node(1)	Node(1)
head->next	null	null	Node(2)	Node(2)	Node(2)
head->next->next	null	null	Node(3)	Node(3)	Node(3)
head->next->next->next	null	null	null	Node(6)	Node(6)
array	[empty]	[1,2,3,4,5]	[1,2,3,4,5]	[1,2,3,4,5]	[1,2,3,4,5]

Key Moments - 3 Insights

Why does accessing an element by index in an array take constant time but not in a linked list?

In the execution_table at steps 2 and 7, array access directly uses the index to get the value instantly. In contrast, linked list traversal (steps 8 and 9) requires moving node by node, which takes more time.

Why is inserting at the end of a linked list slower than adding at the end of an array?

The linked list needs traversal to the last node (step 8) before insertion (step 9), while arrays can add at the end if space is available instantly. This is shown in the pointer changes and visual state.

Why can't arrays easily grow in size like linked lists?

Arrays have fixed size allocated at initialization (step 1), so adding beyond capacity requires creating a new array. Linked lists dynamically allocate nodes as needed (steps 3-6).

Visual Quiz - 3 Questions

Look at the execution_table at step 6, what is the visual state of the linked list?

A 1 -> 2 -> 3 -> null

B 1 -> 2 -> 3 -> 6 -> null

C 1 -> 2 -> null

D 6 -> 1 -> 2 -> 3 -> null

💡 Hint

Check the 'Visual State' column at step 6 in the execution_table.

At which step does the linked list first have three nodes?

A Step 5

B Step 4

C Step 3

D Step 6

💡 Hint

Look at the 'Visual State' column and pointer changes in the execution_table.

If we wanted faster random access, which data structure should we choose?

A Linked List

B Both are equal

C Array

D Neither

💡 Hint

Refer to the access operations in steps 2 and 7 in the execution_table.

Concept Snapshot

Linked List vs Array Decision:
- Arrays: Fixed size, fast random access by index
- Linked Lists: Dynamic size, efficient insertions/deletions
- Choose Array for frequent access
- Choose Linked List for frequent insert/delete
- Arrays need resizing to grow
- Linked Lists use pointers to link nodes

Full Transcript

This visual shows how arrays and linked lists differ in usage. Arrays store elements in continuous memory, allowing fast access by index as seen in steps 2 and 7. Linked lists store elements as nodes linked by pointers, requiring traversal to access or insert elements, shown in steps 3 to 9. Arrays have fixed size, while linked lists can grow dynamically. Choose arrays when you need quick access and linked lists when you need frequent insertions or deletions.

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