单链表的基本操作，参考附件的代码，完成单链表的创建（尾插法和头插法）、输出链表、查找链表的第k个结点、在链表的第k个结点后插入新结点、在链表的第k个结点前插入新结点、删除链表的第k个结点等函数。

单链表是一种线性数据结构，每个节点包含两个部分：数据域和指针域，其中指针域指向下一个节点。以下是单链表的一些基本操作：

1. **链表创建** (尾插法)：

class Node:
    def __init__(self, data=None):
        self.data = data
        self.next = None

def append(head, data):
    if not head:
        head = Node(data)
    else:
        current = head
        while current.next:
            current = current.next
        current.next = Node(data)
    return head

(头插法) 可以类似地实现，但在头部插入节点：

def prepend(head, data):
    new_node = Node(data)
    new_node.next = head
    return new_node

2. **输出链表**：

def print_list(head):
    current = head
    while current:
        print(current.data, end=" -> ")
        current = current.next
    print("None")

3. **查找第k个节点**：

def find_kth_node(head, k):
    count = 0
    current = head
    while current and count < k:
        count += 1
        current = current.next
    if count == k:
        return current
    else:
        return None

4. **在第k个节点后插入新节点**：

def insert_after_kth(head, k, data):
    if not head or k <= 0:
        return None
    node_to_insert = find_kth_node(head, k)
    if not node_to_insert:
        return None
    new_node = Node(data)
    new_node.next = node_to_insert.next
    node_to_insert.next = new_node
    return head

5. **在第k个节点前插入新节点**：

def insert_before_kth(head, k, data):
    if not head or k <= 0:
        return None
    if k == 1:
        return prepend(head, data)
    previous_node = find_kth_node(head, k - 1)
    if previous_node:
        new_node = Node(data)
        new_node.next = previous_node.next
        previous_node.next = new_node
    return head

6. **删除第k个节点**：

def delete_kth_node(head, k):
    if not head or k <= 0:
        return head
    if k == 1:
        return head.next
    current = head
    for _ in range(k - 1):
        current = current.next
    if current is None or current.next is None:
        return head
    current.next = current.next.next
    return head