leetcode445 Add Two Numbers II

"""
You are given two non-empty linked lists representing two non-negative integers. The most significant digit comes first and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.
You may assume the two numbers do not contain any leading zero, except the number 0 itself.
Follow up:
What if you cannot modify the input lists? In other words, reversing the lists is not allowed.
Example:
Input: (7 -> 2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 8 -> 0 -> 7
"""
"""
与leetcode2 类似。
这两题都是自己做出来的
"""
class ListNode:
    def __init__(self, x):
        self.val = x
        self.next = None

class Solution:
    def addTwoNumbers(self, l1, l2):
        num1, num2 = 0, 0
        if l1 and not l2:
            return l1
        if l2 and not l1:
            return l1
        p1, p2 = l1, l2 #将链表l1 和 l2 转成数字num1 和num2 求和得num
        while p1:
            num1 = num1 * 10 + p1.val
            p1 = p1.next
        while p2:
            num2 = num2 * 10 + p2.val
            p2 = p2.next
        num = num1 + num2
        res = ListNode(0)  #头结点
        end = ListNode(num % 10) #尾结点
        res.next = end
        end.next = None
        num = num // 10
        while num:  #从链表尾部插入当前结点cur
            cur = ListNode(num % 10) #当前结点
            res.next = cur
            cur.next = end
            end = cur
            num = num // 10
        return res.next