'''
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]
'''
# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next


# Note: when a pointer enters .next and points to another node, the original .next isn't changed accordingly

class Solution:
    def addTwoNumbers(self, l1: ListNode, l2: ListNode) -> ListNode:
        c = 0
        ans = l1
        last = ListNode(next=l1)

        # merge front
        while l1 and l2:
            l1.val, c = (l1.val + l2.val + c) % 10, (l1.val + l2.val + c) // 10
            l1 = l1.next
            l2 = l2.next
            last = last.next

        # merge back
        if l2:
            last.next = l2
            l1 = l2
        while l1:
            l1.val, c = (l1.val + c) % 10, (l1.val + c) // 10
            l1 = l1.next
            last = last.next

        # rear check
        if c == 1:
            last.next = ListNode(val=1)

        return ans