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

Practice - 5 Tasks

1fill in blank

easy

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

DSA C#

ListNode 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 instead of sum % 10.

Adding 10 to sum instead of taking modulo.

Multiplying sum by 10.

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 for the next addition.

DSA C#

carry = [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 modulo (%) instead of division for carry.

Adding 10 to sum instead of dividing.

Multiplying sum by 10.

Carry is the digit to carry over to the next node, which is sum divided by 10 (integer division).

3fill in blank

hard

Fix the error in the loop condition to continue while either list has nodes or carry is non-zero.

DSA C#

while (l1 != null || l2 != null || [1] != 0) {

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

Acarry

Bsum

CnewNode

Dresult

Attempts:

3 left

💡 Hint

Common Mistakes

Using sum instead of carry in the condition.

Using node variables instead of carry.

Not including carry in the loop condition.

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

4fill in blank

hard

Fill both blanks to correctly move to the next nodes in the lists.

DSA C#

l1 = l1 != null ? [1] : null;
l2 = l2 != null ? [2] : null;

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Al1.next

Bl2.next

Cl1

Dl2

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning l1 or l2 directly without .next.

Swapping l1.next and l2.next.

Not checking for null before moving.

We move l1 and l2 to their next nodes if they are not null.

5fill in blank

hard

Fill all three blanks to build the linked list and return the result.

DSA C#

ListNode dummyHead = new ListNode(0);
ListNode current = dummyHead;

while (l1 != null || l2 != null || carry != 0) {
    int x = (l1 != null) ? l1.val : 0;
    int y = (l2 != null) ? l2.val : 0;
    int sum = x + y + carry;
    carry = sum / 10;
    current.next = new ListNode([1]);
    current = [2];
    l1 = (l1 != null) ? l1.next : null;
    l2 = (l2 != null) ? l2.next : null;
}

return [3];

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

Asum % 10

Bcurrent.next

CdummyHead.next

Dcurrent

Attempts:

3 left

💡 Hint

Common Mistakes

Returning dummyHead instead of dummyHead.next.

Assigning current incorrectly.

Using sum instead of sum % 10 for node value.

We create a new node with sum % 10, move current to current.next, and return dummyHead.next as the head of the result list.

Practice

1. What does the problem "Add Two Numbers Represented as Linked List" mainly involve?

easy

A. Sorting two linked lists in ascending order

B. Adding digits node by node from two linked lists representing numbers

C. Finding the middle node of a linked list

D. Reversing a linked list

Solution

1. Step 1: Understand the problem context

   The problem involves two linked lists where each node contains a single digit. These digits represent numbers in reverse order.

2. Step 2: Identify the required operation

   We need to add these two numbers digit by digit, managing carry, and return the sum as a linked list.

3. Final Answer:

   Adding digits node by node from two linked lists representing numbers -> Option B

4. Quick Check:

   Adding digits node by node = B [OK]

Hint: Focus on digit-wise addition with carry [OK]

Common Mistakes:

• Confusing addition with sorting
• Ignoring carry during addition
• Assuming numbers are stored in forward order

2. Which of the following C# code snippets correctly initializes a linked list node for this problem?

easy

A. ListNode node = new ListNode();

B. ListNode node = ListNode(5);

C. ListNode node = new ListNode(5);

D. ListNode node = new ListNode.val(5);

Solution

1. Step 1: Recall C# object initialization syntax

   In C#, to create an object, use the 'new' keyword followed by the constructor call with parameters.

2. Step 2: Check each option for correct syntax

   A creates node with default val=0, incorrect for val=5. B misses 'new'. C: ListNode node = new ListNode(5); is correct. D has invalid syntax.

3. Final Answer:

   ListNode node = new ListNode(5); -> Option C

4. Quick Check:

   Use 'new' keyword with constructor = C [OK]

Hint: Use 'new' keyword to create objects in C# [OK]

Common Mistakes:

• Omitting 'new' keyword
• Incorrect constructor call syntax
• Trying to assign value directly without constructor

3. What will be the output linked list after adding these two numbers represented as linked lists?

List1: 2 -> 4 -> 3 -> null (represents 342)
List2: 5 -> 6 -> 4 -> null (represents 465)

Code snippet:

var result = AddTwoNumbers(l1, l2);
PrintList(result);

What is the printed linked list?

medium

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

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

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

D. 2 -> 4 -> 3 -> 5 -> 6 -> 4 -> null

Solution

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

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

2. Step 2: Form the resulting linked list

   The result is 7 -> 0 -> 8 -> null, representing 807.

3. Final Answer:

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

4. Quick Check:

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

Hint: Add digits with carry, store result digit-wise [OK]

Common Mistakes:

• Not handling carry correctly
• Appending nodes instead of summing digits
• Confusing order of digits

4. Identify the error in this C# method snippet for adding two numbers represented as linked lists:

public ListNode AddTwoNumbers(ListNode l1, ListNode l2) {
    int carry = 0;
    ListNode dummy = new ListNode(0);
    ListNode current = dummy;
    while (l1 != null || l2 != null) {
        int x = (l1 != null) ? l1.val : 0;
        int y = (l2 != null) ? l2.val : 0;
        int sum = x + y + carry;
        carry = sum / 10;
        current.next = new ListNode(sum % 10);
        current = current.next;
        if (l1 != null) l1 = l1.next;
        if (l2 != null) l2 = l2.next;
    }
    return dummy.next;
}

medium

A. Returning dummy instead of dummy.next

B. Incorrect initialization of carry variable

C. Wrong use of ternary operator for x and y

D. Missing check to add carry node after loop ends

Solution

1. Step 1: Analyze loop and carry handling

   The loop adds digits and updates carry, but after loop ends, if carry > 0, it must be added as a new node.

2. Step 2: Identify missing carry node addition

   Code does not add a node for leftover carry after loop, which causes incorrect result for sums like 5+5=10.

3. Final Answer:

   Missing check to add carry node after loop ends -> Option D

4. Quick Check:

   Carry node after loop missing = A [OK]

Hint: Always add leftover carry node after loop [OK]

Common Mistakes:

• Forgetting to add carry node after loop
• Returning dummy instead of dummy.next
• Incorrect carry initialization

5. Given two linked lists representing numbers in reverse order:
List1: 9 -> 9 -> 9 -> 9 -> null (represents 9999)
List2: 1 -> null (represents 1)

What will be the resulting linked list after adding them using the standard algorithm?

hard

A. 0 -> 0 -> 0 -> 0 -> 1 -> null

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

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

D. 9 -> 9 -> 9 -> 9 -> 1 -> null

Solution

1. Step 1: Add digits with carry step-by-step

   9+1=10 (digit 0, carry 1), then 9+0+1=10 (digit 0, carry 1) repeated for next two nodes.

2. Step 2: Add final carry node

   After last addition, carry is 1, so add new node with value 1 at the end.

3. Final Answer:

   0 -> 0 -> 0 -> 0 -> 1 -> null -> Option A

4. Quick Check:

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

Hint: Carry can extend list length by one node [OK]

Common Mistakes:

• Ignoring carry after last node
• Appending digits without carry
• Misplacing the carry node