【leetcode】445. 两数相加 II（python）

方法：链表反转后相加

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def reverse(self, head):   # 链表反转
        pre = head
        cur = head.next
        pre.next = None
        while cur:
            tmp = cur.next
            cur.next = pre
            pre = cur
            cur = tmp
        return pre
    def addTwoNumbers(self, l1: ListNode, l2: ListNode) -> ListNode:
        l1 = self.reverse(l1)
        l2 = self.reverse(l2)

        h = ListNode((l1.val + l2.val) % 10)   # 额外记下头结点，方便后续反转链表
        flag = (l1.val + l2.val) // 10
        p = h
        l1 = l1.next
        l2 = l2.next
        while l1 or l2 or flag:
            if l1 and l2:
                node = ListNode((l1.val + l2.val + flag) % 10)
                flag = (l1.val + l2.val + flag) // 10
                l1 = l1.next
                l2 = l2.next
            elif l1:
                node = ListNode((l1.val + flag) % 10)
                flag = (l1.val + flag) // 10
                l1 = l1.next
            elif l2:
                node = ListNode((l2.val + flag) % 10)
                flag = (l2.val + flag) // 10
                l2 = l2.next
            elif flag:
                node = ListNode(flag)
                flag = 0
            p.next = node
            p = node
           
        return self.reverse(h)