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

Practice - 5 Tasks

1fill in blank

easy

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

DSA Go

newNode := &ListNode{Val: [1], Next: nil}

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 instead of sum % 10, which gives the carry instead of the digit.

Adding 10 to sum which changes the value incorrectly.

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

2fill in blank

medium

Complete the code to update the carry after adding two digits.

DSA Go

carry = [1]

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Asum / 10

Bsum * 10

Csum % 10

Dsum + 10

Attempts:

3 left

💡 Hint

Common Mistakes

Using modulo operator instead of division for carry.

Multiplying sum by 10 which is incorrect.

Carry is the part of sum that goes to the next digit, which is sum divided by 10.

3fill in blank

hard

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

DSA Go

for l1 != nil || l2 != nil || [1] != 0 {

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Acarry

Bsum

Cnext

DnewNode

Attempts:

3 left

💡 Hint

Common Mistakes

Using sum instead of carry in the loop condition.

Using node pointers instead of carry.

We continue the loop while carry is not zero to add the leftover carry digit.

4fill in blank

hard

Fill both blanks to add values from nodes or zero if node is nil.

DSA Go

x := 0
if l1 != nil {
    x = [1]
}
y := 0
if l2 != nil {
    y = [2]
}

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

Al1.Val

Bl2.Val

Cl1.Next

Dl2.Next

Attempts:

3 left

💡 Hint

Common Mistakes

Using Next instead of Val which is a pointer, not a value.

Not checking for nil before accessing Val.

We assign x and y the values of the current nodes if they exist, otherwise zero.

5fill in blank

hard

Fill all three blanks to move to the next nodes and link the new node.

DSA Go

current.Next = newNode
current = [1]
if l1 != nil {
    l1 = [2]
}
if l2 != nil {
    l2 = [3]
}

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

Acurrent.Next

Bl1.Next

Cl2.Next

DnewNode.Next

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning current to newNode instead of current.Next.

Using newNode.Next instead of l1.Next or l2.Next.

Not advancing l1 or l2 pointers correctly.

We move current to current.Next, and move l1 and l2 to their Next nodes if they exist.

Practice

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

easy

A. Reverse the digits of a number stored in a linked list.

B. Multiply two numbers stored as linked lists and return the product as a linked list.

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

D. Find the maximum digit in a linked list representing a number.

Solution

1. Step 1: Understand the problem statement and identify the expected output

   The problem requires adding two numbers where each number is represented by a linked list, with each node holding a single digit. The sum should also be returned as a linked list, where each node contains a digit of the result.

2. Final Answer:

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

3. Quick Check:

   Adding digits node by node = Add two numbers where each digit is stored in a node of a linked list, and return the sum as a linked list. [OK]

Hint: Focus on adding digit by digit with carry [OK]

Common Mistakes:

• Confusing addition with multiplication
• Ignoring carry over between digits
• Returning a number instead of a linked list

2. Which of the following Go code snippets correctly defines a node for a singly linked list used to represent a digit in the "Add Two Numbers Represented as Linked List" problem?

easy

A. type ListNode struct { Val int; Next *ListNode }

B. type ListNode struct { Value string; Next ListNode }

C. type Node struct { Val int; Next int }

D. type ListNode struct { Val int; Next ListNode }

Solution

1. Step 1: Check struct field types and verify pointer usage

   The node should hold an integer value and a pointer to the next node, so Val is int and Next is *ListNode. Next must be a pointer to ListNode to allow linking nodes dynamically.

2. Final Answer:

   type ListNode struct { Val int; Next *ListNode } -> Option A

3. Quick Check:

   Pointer to next node = type ListNode struct { Val int; Next *ListNode } [OK]

Hint: Use *ListNode for next pointer in Go structs [OK]

Common Mistakes:

• Using value type instead of pointer for Next
• Using wrong field types like string for digits
• Missing semicolons or commas in struct

3. What will be 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) Use the standard addition approach digit by digit with carry.

medium

A. 7 -> 0 -> 8 -> null

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

C. 7 -> 10 -> 7 -> null

D. 7 -> 1 -> 8 -> null

Solution

1. Step 1: Add digits from head nodes with carry and form the result linked list

   2 + 5 = 7, carry 0; next digits 4 + 6 = 10, store 0 carry 1; next digits 3 + 4 + 1 = 8. The result digits are 7 -> 0 -> 8 representing 807.

2. Final Answer:

   7 -> 0 -> 8 -> null -> Option A

3. Quick Check:

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

Hint: Add digits with carry, store remainder as node value [OK]

Common Mistakes:

• Not carrying over when sum >= 10
• Adding digits in wrong order
• Confusing node values with actual number order

4. Consider this Go function snippet to add two numbers represented as linked lists. What is the bug? func addTwoNumbers(l1 *ListNode, l2 *ListNode) *ListNode { carry := 0 dummy := &ListNode{} current := dummy for l1 != nil && l2 != nil { sum := l1.Val + l2.Val + carry carry = sum / 10 current.Next = &ListNode{Val: sum % 10} current = current.Next l1 = l1.Next l2 = l2.Next } return dummy.Next }

medium

A. The dummy node is not initialized properly.

B. Carry is not calculated correctly after sum.

C. The function does not return the correct node.

D. The loop stops when either list ends, missing remaining nodes in longer list.

Solution

1. Step 1: Analyze loop condition and identify missing processing

   The loop runs only while both l1 and l2 are not nil, so it stops if one list is longer. Remaining nodes in the longer list and any carry after loop are not handled, causing incorrect results.

2. Final Answer:

   The loop stops when either list ends, missing remaining nodes in longer list -> Option D

3. Quick Check:

   Loop condition misses longer list nodes = The loop stops when either list ends, missing remaining nodes in longer list. [OK]

Hint: Use OR condition to continue until both lists and carry are done [OK]

Common Mistakes:

• Using AND instead of OR in loop condition
• Ignoring leftover carry after loop
• Not linking remaining nodes after loop

5. You have two linked lists representing numbers in reverse order: List 1: 9 -> 9 -> 9 -> 9 -> null (represents 9999) List 2: 1 -> null (represents 1) What will be the resulting linked list after adding these two numbers?

hard

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

B. 0 -> 0 -> 0 -> 0 -> 1 -> null

C. 1 -> 0 -> 0 -> 0 -> 0 -> null

D. 9 -> 9 -> 9 -> 10 -> null

Solution

1. Step 1: Add digits with carry step-by-step and add final carry as new node

   9+1=10, store 0 carry 1; next 9+0+1=10, store 0 carry 1; repeat for all 9s, each time store 0 carry 1. After last 9, carry is 1, so add a new node with value 1 at the end.

2. Final Answer:

   0 -> 0 -> 0 -> 0 -> 1 -> null -> Option B

3. Quick Check:

   9999 + 1 = 10000 = 0->0->0->0->1 [OK]

Hint: Remember to add carry node if leftover after last digit [OK]

Common Mistakes:

• Not adding extra node for leftover carry
• Stopping addition too early
• Misplacing the carry node in the list