Add Two Numbers Represented as Linked List in DSA Python - Build from Scratch

Add Two Numbers Represented as Linked List

📖 Scenario: You are working on a simple calculator app that adds two very large numbers. These numbers are stored in reverse order, digit by digit, in two linked lists. Each node contains a single digit. Your task is to add these two numbers and return the sum as a linked list in the same reversed order.

🎯 Goal: Build a program that creates two linked lists representing numbers, adds them digit by digit, and prints the resulting linked list showing the sum.

📋 What You'll Learn

Create a linked list node class called ListNode with attributes val and next

Create two linked lists representing the numbers 342 and 465 (stored in reverse order)

Write a function addTwoNumbers that takes the two linked lists and returns their sum as a linked list

Print the resulting linked list in the format: 7 -> 0 -> 8 -> None

💡 Why This Matters

🌍 Real World

Adding large numbers digit by digit is useful in calculators, financial software, and systems that handle numbers too large for standard data types.

💼 Career

Understanding linked lists and how to manipulate them is important for software engineering roles, especially those involving data structures, algorithms, and system design.

1

Create the ListNode class and two linked lists

Create a class called ListNode with an __init__ method that takes val and next (default None). Then create two linked lists: l1 representing the number 342 as 2 -> 4 -> 3 -> None and l2 representing the number 465 as 5 -> 6 -> 4 -> None.

DSA Python

# Define ListNode class and create linked lists l1 and l2
# Your code here

Hint

Think of each digit as a node. The first node holds the ones place, the second the tens, and so on.

2

Create a helper function to print linked lists

Create a function called print_linked_list that takes a ListNode and prints its values in the format: val1 -> val2 -> val3 -> None.

DSA Python

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

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

# Define print_linked_list function below
# Your code here

Hint

Use a loop to go through each node until you reach the end (None).

3

Write the addTwoNumbers function

Write a function called addTwoNumbers that takes two linked lists l1 and l2. Add the numbers digit by digit, handling carry, and return the sum as a new linked list.

DSA Python

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

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

def print_linked_list(node):
    while node:
        print(node.val, end=' -> ')
        node = node.next
    print('None')

# Define addTwoNumbers function below
# Your code here

Hint

Use a dummy node to build the result list. Add digits and carry until all nodes and carry are processed.

4

Print the sum linked list

Call addTwoNumbers with l1 and l2, store the result in result, then use print_linked_list(result) to print the sum linked list.

DSA Python

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

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

def print_linked_list(node):
    while node:
        print(node.val, end=' -> ')
        node = node.next
    print('None')

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

# Call addTwoNumbers and print the result below
# Your code here

Hint

The sum of 342 and 465 is 807, which should be printed as 7 -> 0 -> 8 -> None.