链表：两数之和

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

class Solution:

    def addTwoNumbers(self, l1: ListNode, l2: ListNode) -> ListNode:
    #设置初始头节点
        dummy = p = ListNode(None)
        #初始节点和初始化0
        s = 0
        #同时循环遍历两个链表
        while l1 or l2 or s != 0:
            s += (l1.val if l1 else 0) + (l2.val if l2 else 0)
            #判断是否进位
            p.next = ListNode(s % 10)
            #移动指针
            p = p.next
            if l1: l1 = l1.next
            if l2: l2 = l2.next
            #更新是否进位（1 or 0）
            s = s // 10

        return dummy.next