请实现完整的结点定义、单链表的插入和删除结点函数定义及调用

在Python中，我们可以定义一个简单的节点类`Node`，然后创建单链表相关的插入(`insert`)和删除(`delete`)操作。这里是一个基础的实现：

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

# 插入节点到链表的头部
def insert_at_head(head, data):
    new_node = Node(data)
    new_node.next = head
    return new_node

# 插入节点到链表的尾部
def insert_at_tail(head, data):
    if not head:
        return Node(data)
    
    current = head
    while current.next:
        current = current.next
    current.next = Node(data)
    return head

# 删除指定值的节点（如果存在）
def delete_node(head, value):
    if not head:
        return head
    
    if head.data == value:
        return head.next
    
    previous = head
    current = head.next
    while current:
        if current.data == value:
            previous.next = current.next
            return head
        previous = current
        current = current.next
    
    # 如果值没找到，返回原链表
    return head

# 示例调用
head = None
head = insert_at_head(head, 5)  # 将5添加到链表头部
head = insert_at_tail(head, 7)  # 将7添加到链表尾部

# 删除值为5的节点
new_head = delete_node(head, 5)

#