add two number.py

#=========================================================#
# leetcode.com problems 
# 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.
#=========================================================#

from typing import Optional


# 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()
        cur = dummy
        
        carry = 0
        while l1 or l2 or carry:
            val1 = l1.val if l1 else 0
            val2 = l2.val if l2 else 0
            
            val = val1 + val2 + carry
            
            carry = val // 10
            cur.next = ListNode(val % 10)
            
            cur = cur.next
            l1 = l1.next if l1 else None
            l2 = l2.next if l2 else None
        
        return dummy.next