第二题，两数相加

递归实现

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]:
        # 都结束了末尾节点就为None
        if l1 == None and l2 == None:
            return None
        # 补齐 [9,9,9,9] + [9] 的情况
        if l1 == None:
            l1=ListNode(0)
        if l2 == None:
            l2= ListNode(0)
        # 值加
        sum = l1.val + l2.val
        if sum >= 10:
            # 判断[9] + [9]的情况，进位
            if l1.next == None:
                l1.next = ListNode(0)
            l1.next.val +=1
            return ListNode(sum % 10 ,self.addTwoNumbers(l1.next, l2.next))
        return ListNode(sum, self.addTwoNumbers(l1.next, l2.next))