下面关于单链表和双链表的描述中，正确的是（ ）。

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

def delete_dnode(node):
    if node.prev:
        node.prev.next = node.next
    if node.next:
        node.next.prev = node.prev
    node.prev = None
    node.next = None

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

def delete_snode(head, node):
    if head is None or node is None:
        return
    prev = head
    while prev.next != node:
        prev = prev.next
    prev.next = node.next
    node.next = None