算法: 链表模仿整数相加2. Add Two Numbers

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.

链表解法，注意边界进位和最后一个进位

# 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]:
        result = result_head = ListNode(-1)
        carry = 0
        while l1 != None or l2 != None or carry != 0:
            l1_val = l1.val if l1 else 0
            l2_val = l2.val if l2 else 0
            new_val = l1_val + l2_val + carry
            carry = new_val // 10
            result.next = ListNode(new_val % 10)
            result = result.next
            l1 = l1.next if l1 else None
            l2 = l2.next if l2 else None

        return result_head.next