2.py

class Solution:
    def addTwoNumbers(self, l1: Optional[ListNode], l2: Optional[ListNode]) -> Optional[ListNode]:
        dummy = ListNode(0)
        
        currentNode = dummy
        extra = 0
        
        while l1 or l2:
            currentSum = extra
            
            if l1:
                currentSum += l1.val
                l1 = l1.next
                
            if l2:
                currentSum += l2.val
                l2 = l2.next
                
            currentNode.next = ListNode(currentSum % 10)
            currentNode = currentNode.next
            extra = currentSum // 10
            
        if extra > 0:
            currentNode.next = ListNode(extra)
        
        return dummy.next