双向链表创建与遍历

def solve():
    class DNode:
        def __init__(____):
            self.data = data
            self.prev = None
            self.next = None
    head = None
    tail = None
    for val in [____, ____, ____]:
        node = DNode(____)
        if head is None:
            head = tail = node
        else:
            tail.next = node
            node.prev = tail
            tail = node
    cur = head
    while cur:
        print(____)
        cur = cur.next
    cur = tail
    while cur:
        print(____)
        cur = cur.prev