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.

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def addTwoNumbers(self, l1: Optional[ListNode], l2: Optional[ListNode]) -> Optional[ListNode]:
        dummy = ListNode()
        carry = 0
        cur, cur1, cur2 = dummy, l1, l2
        while cur1 is not None or cur2 is not None:
            val1 = cur1.val if cur1 is not None else 0
            val2 = cur2.val if cur2 is not None else 0

            total = val1 + val2 + carry
            val = total % 10
            carry = total // 10

            cur.next = ListNode(val)
            cur = cur.next

            cur1 = cur1.next if cur1 is not None else None
            cur2 = cur2.next if cur2 is not None else None

        if carry != 0:
            cur.next = ListNode(carry)
        
        return dummy.next