链表相加（包括正向和反向）

1、正向

# 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: ListNode, l2: ListNode) -> ListNode:
        s1 = ''
        s2 = ''
        while l1:
            s1 = s1 + str(l1.val)
            l1 = l1.next
        while l2:
            s2 = s2 + str(l2.val)
            l2 = l2.next
        Str = str(int(s1) + int(s2))
        lens = len(Str)
        node = ListNode(int(Str[0]))
        cur = node
        for i in range(1,lens):
            node.next = ListNode(int(Str[i]))
            node = node.next
        return cur

2、反向

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

class Solution:
    def addTwoNumbers(self, l1: ListNode, l2: ListNode) -> ListNode:
        head = ListNode(0)
        carry = 0
        node = head
        while l1 or l2:
            if not l1:
                node.next = l2
                l1 = ListNode(0)
            if not l2:
                node.next = l1
                l2 = ListNode(0)
            carry = carry + l1.val + l2.val
            node.next = ListNode(carry%10)
            node = node.next
            carry =  carry //10
            l1 = l1.next
            l2 = l2.next
        if carry:
            node.next = ListNode(carry)
        return head.next