'''
2. Add Two Numbers.

You are given two non-empty linked lists representing two non-negative integers. The digits are stored in 
reverse order, and each of their nodes contains a single digit. Add the two numbers and return the sum as a linked list.

You may assume the two numbers do not contain any leading zero, except the number 0 itself. 

Example 1:

Input: l1 = [2,4,3], l2 = [5,6,4]
Output: [7,0,8]
Explanation: 342 + 465 = 807.

Example 2:

Input: l1 = [0], l2 = [0]
Output: [0]
Example 3:

Input: l1 = [9,9,9,9,9,9,9], l2 = [9,9,9,9]
Output: [8,9,9,9,0,0,0,1]

Constraints:

The number of nodes in each linked list is in the range [1, 100].
0 <= Node.val <= 9
It is guaranteed that the list represents a number that does not have leading zeros.
'''

class ListNode:

    def __init__(self, val : int ):
        self.val = val
        self.next = None

def build_list(values : list[int] ) -> ListNode:

    sentinel = ListNode(0)
    node = sentinel

    for val in values:
        node.next = ListNode(val)
        node = node.next

    return sentinel.next

def to_vector(head : ListNode ) -> list[int]:

    node, result = head, []

    while node:
        result.append(node.val)
        node = node.next

    return result

def addTwoNumbers(l1: ListNode, l2: ListNode) -> ListNode:
    
    sentinel = ListNode(0)
    node, carry = sentinel, 0
    
    while l1 or l2 or carry:
        
        left = l1.val if l1 else 0
        right = l2.val if l2 else 0
        
        val = left + right + carry
        
        node.next = ListNode(val % 10)
        node = node.next
        
        carry = 1 if val > 9 else 0
        
        l1, l2 = l1.next if l1 else None, l2.next if l2 else None
        
    return sentinel.next