Add Two Numbers Represented as Linked List in DSA Javascript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a new node with the sum value.

DSA Javascript

let newNode = new ListNode([1]);

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

Asum - 10

Bsum % 10

Csum * 10

Dsum + 10

Attempts:

3 left

💡 Hint

Common Mistakes

Using sum + 10 or sum * 10 instead of modulo causes wrong digit storage.

Forgetting to use modulo leads to nodes storing numbers greater than 9.

We use sum % 10 to get the digit to store in the node because each node stores a single digit.

2fill in blank

medium

Complete the code to update the carry for the next addition.

DSA Javascript

carry = [1];

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AMath.floor(sum / 10)

Bsum % 10

Csum * 10

Dsum + 1

Attempts:

3 left

💡 Hint

Common Mistakes

Using sum % 10 for carry is wrong because it gives the digit, not the carry.

Using sum * 10 or sum + 1 causes incorrect carry values.

Carry is the tens digit of sum, so we use Math.floor(sum / 10) to get it.

3fill in blank

hard

Fix the error in the loop condition to continue while nodes or carry exist.

DSA Javascript

while (l1 !== null || [1] !== null || carry !== 0) {

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Asum

Bl1

Ccarry

Dl2

Attempts:

3 left

💡 Hint

Common Mistakes

Checking l1 twice instead of l2 causes missing nodes from second list.

Checking carry or sum incorrectly in place of l2 causes logic errors.

We check l2 !== null to continue if the second list still has nodes.

4fill in blank

hard

Fill both blanks to safely get values from nodes or use 0 if null.

DSA Javascript

let val1 = l1 ? [1] : 0;
let val2 = l2 ? [2] : 0;

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Al1.val

Bl2.val

Cl1.next

Dl2.next

Attempts:

3 left

💡 Hint

Common Mistakes

Using .next instead of .val returns the next node, not the value.

Not checking for null nodes causes runtime errors.

We get the value from l1.val and l2.val if nodes exist, else 0.

5fill in blank

hard

Fill all three blanks to move to the next nodes or null safely.

DSA Javascript

l1 = l1 ? [1] : null;
l2 = l2 ? [2] : null;
current = current.[3];

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Anext

Bval

Dprev

Attempts:

3 left

💡 Hint

Common Mistakes

Using .val instead of .next moves to value, not node.

Using .prev is wrong because list is singly linked.

We move l1 and l2 to their next nodes, and current to current.next to build the list.

Practice

1. What does the problem "Add Two Numbers Represented as Linked List" ask you to do?

easy

A. Add two numbers where each digit is a node in a linked list, and return the sum as a linked list.

B. Multiply two numbers stored in arrays and return the product as an array.

C. Sort two linked lists and merge them into one sorted list.

D. Find the difference between two numbers represented as linked lists.

Solution

1. Step 1: Understand the problem statement

   The problem requires adding two numbers where each number is represented by a linked list. Each node contains a single digit, and digits are stored in reverse order.

2. Step 2: Identify the expected output

   The output should be a linked list representing the sum of the two numbers, also in reverse order.

3. Final Answer:

   Add two numbers represented as linked lists and return their sum as a linked list -> Option A

4. Quick Check:

   Adding digits node by node = Add linked lists [OK]

Hint: Remember digits are reversed in linked lists [OK]

Common Mistakes:

• Thinking digits are stored in normal order
• Confusing addition with multiplication
• Returning a number instead of a linked list

2. Which of the following is the correct way to create a node for a singly linked list in JavaScript for this problem?

easy

A. function ListNode(val) { this.val = val; this.next = null; }

B. class ListNode { constructor(val) { this.value = val; this.nextNode = null; } }

C. let node = { value: val, next: null };

D. function Node(val) { this.val = val; this.nextNode = null; }

Solution

1. Step 1: Check the standard node structure

   The node should have a property named 'val' for the digit and 'next' for the next node pointer.

2. Step 2: Verify the syntax correctness

   function ListNode(val) { this.val = val; this.next = null; } correctly defines a function constructor with 'val' and 'next' properties initialized properly.

3. Final Answer:

   function ListNode(val) { this.val = val; this.next = null; } -> Option A

4. Quick Check:

   Node with val and next properties = function ListNode(val) { this.val = val; this.next = null; } [OK]

Hint: Use 'val' and 'next' for node properties [OK]

Common Mistakes:

• Using 'value' instead of 'val'
• Using 'nextNode' instead of 'next'
• Incorrect constructor syntax

3. What is the output linked list after adding the two numbers represented by these linked lists? List 1: 2 -> 4 -> 3 -> null (represents 342) List 2: 5 -> 6 -> 4 -> null (represents 465)

function addTwoNumbers(l1, l2) {
  let dummy = new ListNode(0);
  let current = dummy;
  let carry = 0;
  while (l1 || l2 || carry) {
    let sum = carry;
    if (l1) { sum += l1.val; l1 = l1.next; }
    if (l2) { sum += l2.val; l2 = l2.next; }
    carry = Math.floor(sum / 10);
    current.next = new ListNode(sum % 10);
    current = current.next;
  }
  return dummy.next;
}

medium

A. 7 -> 0 -> 7 -> null

B. 7 -> 0 -> 8 -> null

C. 7 -> 1 -> 7 -> null

D. 8 -> 0 -> 7 -> null

Solution

1. Step 1: Add digits node by node with carry

   2+5=7 (carry 0), 4+6=10 (carry 1), 3+4+1=8 (carry 0)

2. Step 2: Form the resulting linked list

   The digits are 7 -> 0 -> 8 representing 807 in reverse order.

3. Final Answer:

   7 -> 0 -> 8 -> null -> Option B

4. Quick Check:

   342 + 465 = 807 = 7->0->8 [OK]

Hint: Add digits with carry, build new nodes [OK]

Common Mistakes:

• Ignoring carry after adding digits
• Mixing digit order in output
• Returning sum as a number, not linked list

4. Identify the bug in the following code snippet for adding two numbers represented as linked lists:

function addTwoNumbers(l1, l2) {
  let dummy = new ListNode(0);
  let current = dummy;
  let carry = 0;
  while (l1 !== null && l2 !== null) {
    let sum = carry + l1.val + l2.val;
    carry = Math.floor(sum / 10);
    current.next = new ListNode(sum % 10);
    current = current.next;
    l1 = l1.next;
    l2 = l2.next;
  }
  return dummy.next;
}

medium

A. The function does not return the dummy node itself.

B. The carry is not calculated correctly using Math.floor.

C. The dummy node should be initialized with value 1 instead of 0.

D. The loop condition should use '||' instead of '&&' to handle lists of different lengths.

Solution

1. Step 1: Analyze the loop condition

   The loop stops when either l1 or l2 becomes null, missing remaining nodes if lists differ in length.

2. Step 2: Correct the loop condition

   Using '||' ensures the loop continues until both lists and carry are fully processed.

3. Final Answer:

   The loop condition should use '||' instead of '&&' to handle lists of different lengths. -> Option D

4. Quick Check:

   Loop condition must cover all nodes = The loop condition should use '||' instead of '&&' to handle lists of different lengths. [OK]

Hint: Use '||' to continue while any list or carry remains [OK]

Common Mistakes:

• Stopping loop too early with '&&'
• Ignoring leftover carry after loop
• Not advancing pointers correctly

5. Given two linked lists representing numbers in reverse order, how would you modify the addition function to handle the case where the input lists have different lengths and the final carry creates a new node? Choose the best approach to ensure correct output for inputs like: List 1: 9 -> 9 -> 9 -> null (represents 999) List 2: 1 -> null (represents 1)

function addTwoNumbers(l1, l2) {
  let dummy = new ListNode(0);
  let current = dummy;
  let carry = 0;
  while (l1 || l2 || carry) {
    let sum = carry;
    if (l1) { sum += l1.val; l1 = l1.next; }
    if (l2) { sum += l2.val; l2 = l2.next; }
    carry = Math.floor(sum / 10);
    current.next = new ListNode(sum % 10);
    current = current.next;
  }
  return dummy.next;
}

hard

A. Add zeros to the shorter list before starting the addition to equalize lengths.

B. Stop the loop when both lists end and ignore carry since it won't affect the sum.

C. Use a loop condition that continues while l1, l2, or carry exists, adding carry as a new node if needed.

D. Only add nodes while both lists have nodes, then append the remaining nodes without addition.

Solution

1. Step 1: Understand the need to handle different lengths and carry

   When lists differ in length, addition must continue until both lists and carry are fully processed.

2. Step 2: Use a loop condition that checks l1, l2, or carry

   This ensures the loop adds all digits and creates a new node if carry remains after lists end.

3. Final Answer:

   Use a loop condition that continues while l1, l2, or carry exists, adding carry as a new node if needed. -> Option C

4. Quick Check:

   Loop until all digits and carry processed = Use a loop condition that continues while l1, l2, or carry exists, adding carry as a new node if needed. [OK]

Hint: Loop while any list or carry remains to add all digits [OK]

Common Mistakes:

• Ignoring carry after lists end
• Trying to pad shorter list manually
• Appending leftover nodes without adding carry