假设 head 不为空，下面是实现单向循环链表在头节点后插入新节点的代码，横线处应填入
class Node:
def __init__(self, val):
self.val = val
self.next = None
def insert_after_head(head, x):
new_node = Node(x)
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new_node.next = head.next head.next = new_node
new_node.next = head head.next = new_node
head.next = new_node new_node.next = head
new_node.next = head.next; head = new_node