Add Two Numbers Represented as Linked List in DSA Python - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Adding Two Linked Lists Representing Numbers

What is the output linked list after adding the two numbers represented by these linked lists?

List 1: 2 -> 4 -> 3 -> null
List 2: 5 -> 6 -> 4 -> null

Note: Each node contains a single digit and digits are stored in reverse order.

DSA Python

class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next

def addTwoNumbers(l1, l2):
    dummy = ListNode()
    current = dummy
    carry = 0
    while l1 or l2 or carry:
        val1 = l1.val if l1 else 0
        val2 = l2.val if l2 else 0
        total = val1 + val2 + carry
        carry = total // 10
        current.next = ListNode(total % 10)
        current = current.next
        if l1:
            l1 = l1.next
        if l2:
            l2 = l2.next
    return dummy.next

# Construct lists
l1 = ListNode(2, ListNode(4, ListNode(3)))
l2 = ListNode(5, ListNode(6, ListNode(4)))

result = addTwoNumbers(l1, l2)
output = []
while result:
    output.append(result.val)
    result = result.next
print(output)

A [7, 10, 7]

B [8, 0, 7]

C [7, 0, 8]

D [5, 10, 7]

💡 Hint

Add digits node by node, remember to carry over if sum >= 10.

Adding 342 + 465 gives 807. Since digits are reversed, output list is 7 -> 0 -> 8 -> null.

❓ Predict Output

intermediate

Result of Adding Unequal Length Linked Lists

What is the output linked list after adding these two numbers?

List 1: 9 -> 9 -> 9 -> 9 -> null
List 2: 1 -> null

Digits are stored in reverse order.

DSA Python

class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next

def addTwoNumbers(l1, l2):
    dummy = ListNode()
    current = dummy
    carry = 0
    while l1 or l2 or carry:
        val1 = l1.val if l1 else 0
        val2 = l2.val if l2 else 0
        total = val1 + val2 + carry
        carry = total // 10
        current.next = ListNode(total % 10)
        current = current.next
        if l1:
            l1 = l1.next
        if l2:
            l2 = l2.next
    return dummy.next

l1 = ListNode(9, ListNode(9, ListNode(9, ListNode(9))))
l2 = ListNode(1)

result = addTwoNumbers(l1, l2)
output = []
while result:
    output.append(result.val)
    result = result.next
print(output)

A [0, 0, 0, 0, 1]

B [0, 0, 0, 1]

C [1, 0, 0, 0, 0]

D [9, 9, 9, 10]

💡 Hint

Remember to carry over after the last node if needed.

9999 + 1 = 10000, so the linked list is 0 -> 0 -> 0 -> 0 -> 1 -> null.

🔧 Debug

advanced

Identify the Error in Linked List Addition Code

What error will this code raise when adding two linked lists?

Code snippet:

def addTwoNumbers(l1, l2):
    dummy = ListNode()
    current = dummy
    carry = 0
    while l1 or l2:
        val1 = l1.val if l1 else 0
        val2 = l2.val if l2 else 0
        total = val1 + val2 + carry
        carry = total // 10
        current.next = ListNode(total % 10)
        current = current.next
        if l1:
            l1 = l1.next
        if l2:
            l2 = l2.next
    if carry:
        current.next = ListNode(carry)
    return dummy.next

A SyntaxError: invalid syntax

B AttributeError: 'NoneType' object has no attribute 'val'

C No error, code runs correctly

D TypeError: unsupported operand type(s) for +: 'int' and 'NoneType'

💡 Hint

Check the while loop condition and what happens when l1 or l2 is None.

The loop condition misses the carry check, so when l1 and l2 become None but carry is non-zero, accessing l1.val or l2.val causes AttributeError.

❓ Predict Output

advanced

Output When Adding Lists with Zero Nodes

What is the output linked list after adding these two numbers?

List 1: 0 -> null
List 2: 0 -> null

Digits are stored in reverse order.

DSA Python

class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next

def addTwoNumbers(l1, l2):
    dummy = ListNode()
    current = dummy
    carry = 0
    while l1 or l2 or carry:
        val1 = l1.val if l1 else 0
        val2 = l2.val if l2 else 0
        total = val1 + val2 + carry
        carry = total // 10
        current.next = ListNode(total % 10)
        current = current.next
        if l1:
            l1 = l1.next
        if l2:
            l2 = l2.next
    return dummy.next

l1 = ListNode(0)
l2 = ListNode(0)

result = addTwoNumbers(l1, l2)
output = []
while result:
    output.append(result.val)
    result = result.next
print(output)

A [0]

B []

C [1]

D [0, 0]

💡 Hint

Adding zero plus zero should give zero.

0 + 0 = 0, so the output list is a single node with 0.

❓ Predict Output

expert

Output of Adding Large Numbers with Multiple Carries

What is the output linked list after adding these two numbers?

List 1: 9 -> 9 -> 9 -> 9 -> 9 -> 9 -> 9 -> null
List 2: 9 -> 9 -> 9 -> 9 -> null

Digits are stored in reverse order.

DSA Python

class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next

def addTwoNumbers(l1, l2):
    dummy = ListNode()
    current = dummy
    carry = 0
    while l1 or l2 or carry:
        val1 = l1.val if l1 else 0
        val2 = l2.val if l2 else 0
        total = val1 + val2 + carry
        carry = total // 10
        current.next = ListNode(total % 10)
        current = current.next
        if l1:
            l1 = l1.next
        if l2:
            l2 = l2.next
    return dummy.next

l1 = ListNode(9, ListNode(9, ListNode(9, ListNode(9, ListNode(9, ListNode(9, ListNode(9)))))))
l2 = ListNode(9, ListNode(9, ListNode(9, ListNode(9))))

result = addTwoNumbers(l1, l2)
output = []
while result:
    output.append(result.val)
    result = result.next
print(output)

A [8, 9, 9, 9, 9, 9, 9, 1]

B [8, 9, 9, 9, 0, 0, 1]

C [0, 9, 9, 9, 0, 0, 0, 1]

D [8, 9, 9, 9, 0, 0, 0, 1]

💡 Hint

Add digit by digit with carry, carefully track carries beyond the shorter list.

9999999 + 9999 = 10009998, reversed digits are 8 -> 9 -> 9 -> 9 -> 0 -> 0 -> 0 -> 1 -> null.