2019-4-7/day2 --add two numbers

题目：给定两个非负整数的链表。数字以相反的顺序存储，每个节点包含一个数字。将这两个数字相加，并作为链表返回

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
Expl

ion: 342 + 465 = 807.

思路：先将两个链表均表示为数字，数字进行相加，再将数字转换为链表输出

直接将链表转置 需要反转  转换为str字符串格式。

中间遇到几个问题，其中一个数为0 时，无需转置。

将字符串进行反转

        num1 = list(num1)
        num1.reverse()
        num1 = "".join(num1)

将两个链表转换为字符串并且进行数字计算。 

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

class Solution(object):
    def addTwoNumbers(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        num1 = ''
        num2 = ''
        while l1:
            num1 += str(l1.val)
            l1 = l1.next
        num1 = list(num1)
        num1.reverse()
        num1 = "".join(num1)
        while l2:
            num2 += str(l2.val)
            l2 = l2.next
        num2 = list(num2)
        num2.reverse()
        num2 = "".join(num2)
        num = str(int(num1) + int(num2))
        if int(num1)==0:
            head = ListNode(num[len(num2)-1])
            a =head
            for i in range(1,len(num2)):
                node = ListNode(num2[len(num2)-i-1])
                head.next =node
                head = head.next
            return a 
        if int(num2)==0:
            head = ListNode(num[len(num1)-1])
            a =head
            for i in range(1,len(num1)):
                node = ListNode(num1[len(num1)-i-1])
                head.next =node
                head = head.next
            return a 
        head = ListNode(num[len(num)-1])
        a =head
        for i in range(1,len(num)):
            node = ListNode(num[len(num)-i-1])
            head.next =node
            head = head.next
        return a