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

Practice - 5 Tasks

1fill in blank

easy

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

DSA Python

new_node = ListNode([1])

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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 modulo

Multiplying sum by 10

The digit to store in the node is the remainder when sum is divided by 10.

2fill in blank

medium

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

DSA Python

carry = [1]

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Asum // 10

Bsum % 10

Csum + 1

Dsum - 10

Attempts:

3 left

💡 Hint

Common Mistakes

Using modulo instead of integer division

Adding 1 to sum instead of dividing

Subtracting 10 from sum

Carry is the quotient when sum is divided by 10.

3fill in blank

hard

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

DSA Python

while l1 or l2 or [1]:

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Anew_node

Bsum

CNone

Dcarry

Attempts:

3 left

💡 Hint

Common Mistakes

Using sum which is undefined here

Using new_node which is a node, not a condition

Using None which stops the loop

The loop must continue if there is a carry left to add.

4fill in blank

hard

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

DSA Python

l1 = l1[1] if l1 else None
l2 = l2[2] if l2 else None

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A.next

B.prev

C.head

D.tail

Attempts:

3 left

💡 Hint

Common Mistakes

Using .prev which is for doubly linked lists backwards

Using .head or .tail which are not node pointers

We move to the next node in each linked list using the .next attribute.

5fill in blank

hard

Fill both blanks to initialize the dummy node, set current pointer, and return the result list.

DSA Python

dummy = ListNode([1])
current = dummy
...
return dummy[2]

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A0

B.next

C.prev

DNone

Attempts:

3 left

💡 Hint

Common Mistakes

Returning dummy instead of dummy.next

Using None instead of 0 for dummy node value

Using .prev which is not correct here

Dummy node starts with 0, current points to dummy, and we return dummy.next as the head of the result list.