Add Two Numbers Represented as Linked List in DSA Go - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does each node in the linked list represent in the problem 'Add Two Numbers Represented as Linked List'?

Each node represents a single digit of a number, stored in reverse order (ones place at the head).

beginner

Why do we add digits starting from the head of the linked lists in this problem?

Because the digits are stored in reverse order, the head contains the least significant digit, so addition starts from there.

beginner

What is the role of the carry variable in adding two numbers represented as linked lists?

The carry stores the overflow digit when the sum of two digits exceeds 9, to be added to the next higher digit.

intermediate

How do you handle linked lists of different lengths when adding two numbers represented as linked lists?

Continue adding digits until both lists are fully processed, treating missing digits as zero.

intermediate

What should you do if there is a carry left after processing all nodes in both linked lists?

Create a new node with the carry value and attach it to the end of the result linked list.

In the problem 'Add Two Numbers Represented as Linked List', what does the head node represent?

A The least significant digit

B The most significant digit

C The middle digit

D The carry digit

The digits are stored in reverse order, so the head node holds the least significant digit.

What do you do when the sum of two digits plus carry is greater than 9?

A Store the full sum in the node

B Ignore the carry

C Store the last digit and carry over the rest

D Restart the addition

You store the last digit (sum % 10) in the node and carry over the rest (sum / 10) to the next addition.

How do you handle when one linked list is longer than the other?

A Stop when the shorter list ends

B Treat missing digits as zero and continue

C Add only the overlapping parts

D Reverse the lists

You treat missing digits as zero and continue adding until both lists are fully processed.

What data structure is used to store the result of adding two numbers represented as linked lists?

A Queue

B Stack

C Array

D Linked list

The result is stored as a linked list in the same reverse order format.

If after adding all digits, carry is still 1, what should you do?

A Add a new node with value 1 at the end

B Subtract 1 from the last node

C Ignore it

D Restart the addition

You add a new node with value 1 to represent the leftover carry.

Explain step-by-step how to add two numbers represented as linked lists.

Think about how you add numbers on paper from right to left.

Describe how to handle carry and different length linked lists when adding two numbers represented as linked lists.

Carry is like the extra you keep in mind when adding digits.

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