Why Linked List Exists and What Problem It Solves in DSA Javascript - Test Your Knowledge

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a linked list node with a value and a pointer to the next node.

DSA Javascript

function ListNode(value) {
  this.value = value;
  this.next = [1];
}

Drag options to blanks, or click blank then click option'

Anull

Bundefined

C0

Dfalse

Attempts:

3 left

💡 Hint

Common Mistakes

Using undefined or 0 instead of null for the next pointer.

The next pointer should start as null because the node does not point to any other node yet.

2fill in blank

medium

Complete the code to add a new node at the end of a linked list.

DSA Javascript

function appendNode(head, value) {
  let newNode = new ListNode(value);
  let current = head;
  while (current.[1] !== null) {
    current = current.next;
  }
  current.next = newNode;
}

Drag options to blanks, or click blank then click option'

Aprev

Bvalue

Cnext

Dhead

Attempts:

3 left

💡 Hint

Common Mistakes

Checking current.value instead of current.next in the loop condition.

We check current.next to find the last node whose next is null.

3fill in blank

hard

Fix the error in the code that tries to insert a node after a given node.

DSA Javascript

function insertAfter(node, value) {
  let newNode = new ListNode(value);
  newNode.next = node.[1];
  node.next = newNode;
}

Drag options to blanks, or click blank then click option'

Aprev

Bhead

Cvalue

Dnext

Attempts:

3 left

💡 Hint

Common Mistakes

Using prev or value instead of next for the new node's next pointer.

To insert after the given node, the new node's next should point to the current node's next.

4fill in blank

hard

Fill both blanks to create a function that counts the number of nodes in a linked list.

DSA Javascript

function countNodes(head) {
  let count = 0;
  let current = head;
  while (current [1] null) {
    count [2] 1;
    current = current.next;
  }
  return count;
}

Drag options to blanks, or click blank then click option'

A!==

B==

C+=

D-=

Attempts:

3 left

💡 Hint

Common Mistakes

Using == instead of !== in the loop condition.

Using -= instead of += to update count.

The loop continues while current is not null, and count increases by 1 each step.

5fill in blank

hard

Fill all three blanks to create a function that finds a node with a given value in a linked list.

DSA Javascript

function findNode(head, target) {
  let current = head;
  while (current !== null) {
    if (current.[1] === [2]) {
      return current;
    }
    current = current.[3];
  }
  return null;
}

Drag options to blanks, or click blank then click option'

Avalue

Btarget

Cnext

Dprev

Attempts:

3 left

💡 Hint

Common Mistakes

Using prev instead of next to move forward.

Comparing to a wrong variable instead of target.

We compare the current node's value to the target, and move to the next node if not found.

Practice

1. Why do we use a linked list instead of an array in some cases?

easy

A. Because linked lists allow easy insertion and deletion without shifting elements

B. Because linked lists use less memory than arrays always

C. Because linked lists store data in contiguous memory locations

D. Because linked lists are faster for accessing elements by index

Solution

1. Step 1: Understand array limitations

   Arrays store elements in contiguous memory, so inserting or deleting in the middle requires shifting elements, which is slow.

2. Step 2: Understand linked list advantages

   Linked lists store elements in nodes linked by pointers, so insertion and deletion can be done by changing pointers without shifting.

3. Final Answer:

   Because linked lists allow easy insertion and deletion without shifting elements -> Option A

4. Quick Check:

   Insertion/deletion ease = B [OK]

Hint: Linked lists excel at insert/delete without moving all elements [OK]

Common Mistakes:

• Thinking linked lists always use less memory
• Believing linked lists store data contiguously
• Assuming linked lists have fast random access

2. Which of the following is the correct way to define a node in a singly linked list in JavaScript?

easy

A. function Node(value) { this.next = value; this.value = null; }

B. function Node(value) { this.value = value; this.prev = null; }

C. function Node(value) { this.value = null; this.next = value; }

D. function Node(value) { this.value = value; this.next = null; }

Solution

1. Step 1: Identify node properties for singly linked list

   A node should store its value and a pointer to the next node, usually called 'next'.

2. Step 2: Check each option's correctness

   function Node(value) { this.value = value; this.next = null; } correctly assigns value and sets next to null initially. Options A, B, and D misuse 'prev' or swap value and next.

3. Final Answer:

   function Node(value) { this.value = value; this.next = null; } -> Option D

4. Quick Check:

   Node with value and next = C [OK]

Hint: Node must have value and next pointer for singly linked list [OK]

Common Mistakes:

• Using 'prev' instead of 'next' for singly linked list
• Swapping value and next assignments
• Not initializing next to null

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

function Node(value) {
  this.value = value;
  this.next = null;
}

const head = new Node(1);
head.next = new Node(2);
head.next.next = new Node(3);

let current = head;
let result = '';
while(current) {
  result += current.value + ' -> ';
  current = current.next;
}
result += 'null';
console.log(result);

medium

A. 1 -> 2 -> 3 -> null

B. 1 -> 2 -> null

C. null -> 1 -> 2 -> 3

D. 3 -> 2 -> 1 -> null

Solution

1. Step 1: Build the linked list nodes

   Nodes with values 1, 2, and 3 are created and linked: head -> 1 -> 2 -> 3 -> null.

2. Step 2: Traverse and build output string

   The while loop appends each node's value followed by ' -> ' until current is null, then adds 'null'.

3. Final Answer:

   1 -> 2 -> 3 -> null -> Option A

4. Quick Check:

   Traverse nodes 1,2,3 then null = A [OK]

Hint: Traverse nodes from head, append values with arrows, end with null [OK]

Common Mistakes:

• Stopping traversal too early
• Appending 'null' at start instead of end
• Reversing node order in output

4. The following code tries to insert a new node with value 4 after the head node in a linked list. What is the error?

function Node(value) {
  this.value = value;
  this.next = null;
}

const head = new Node(1);
head.next = new Node(2);

const newNode = new Node(4);
newNode.next = head.next;
head = newNode;

medium

A. newNode.next should be null instead of head.next

B. Cannot assign to 'head' because it is a constant

C. Missing function to insert node

D. Node constructor is incorrect

Solution

1. Step 1: Identify variable declaration

   head is declared with const, so it cannot be reassigned to a new value.

2. Step 2: Check the reassignment line

   The line 'head = newNode;' tries to assign a new object to head, causing an error.

3. Final Answer:

   Cannot assign to 'head' because it is a constant -> Option B

4. Quick Check:

   Reassigning const variable = A [OK]

Hint: const variables cannot be reassigned after declaration [OK]

Common Mistakes:

• Thinking newNode.next should be null
• Assuming missing insert function causes error
• Believing Node constructor is wrong

5. You want to implement a playlist where songs can be added or removed anywhere efficiently. Which data structure best solves this problem and why?

hard

A. Array, because it allows fast random access to songs

B. Queue, because it follows first-in-first-out order

C. Linked list, because it allows efficient insertions and deletions anywhere

D. Stack, because it follows last-in-first-out order

Solution

1. Step 1: Understand playlist operations

   Adding or removing songs anywhere means insertions and deletions at arbitrary positions.

2. Step 2: Compare data structures

   Arrays have slow insert/delete in middle due to shifting. Stacks and queues have restricted access order. Linked lists allow efficient insert/delete by changing pointers.

3. Final Answer:

   Linked list, because it allows efficient insertions and deletions anywhere -> Option C

4. Quick Check:

   Efficient insert/delete anywhere = D [OK]

Hint: Use linked list for efficient insert/delete anywhere [OK]

Common Mistakes:

• Choosing array for fast random access but ignoring insert/delete cost
• Confusing stack or queue with general insert/delete needs
• Ignoring linked list pointer advantages