链表从链表尾到链表头逆序输出

以下是两种Python实现链表逆序输出的方法：

1. 借助一个新的空链表

class ListNode:
    def __init__(self, x):
        self.val = x
        self.next = None

class Solution:
    def reverseList(self, head: ListNode) -> ListNode:
        new_head = None
        while head:
            temp = head.next
            head.next = new_head
            new_head = head
            head = temp
        return new_head

# 测试
node1 = ListNode(1)
node2 = ListNode(2)
node3 = ListNode(3)
node1.next = node2
node2.next = node3
s = Solution()
new_head = s.reverseList(node1)
while new_head:
    print(new_head.val)
    new_head = new_head.next

2. 在原链表上直接逆序

class ListNode:
    def __init__(self, x):
        self.val = x
        self.next = None

class Solution:
    def reverseList(self, head: ListNode) -> ListNode:
        if not head or not head.next:
            return head
        last = None
        while head:
            temp = head.next
            head.next = last
            last = head
            head = temp
        return last

# 测试
node1 = ListNode(1)
node2 = ListNode(2)
node3 = ListNode(3)
node1.next = node2
node2.next = node3
s = Solution()
new_head = s.reverseList(node1)
while new_head:
    print(new_head.val)
    new_head = new_head.next