链表！链表！
链表！链表！
参考图中的链表
写出单链表插入和删除一个节点的伪代码算法
插入一个节点
x = p -> next
p -> next = q
q -> next = x
删除一个节点
x = p -> next
p -> next = x -> next;
free(x)
x = p -> next
p -> next = q
q -> next = x
x = p -> next
p -> next = x -> next;
free(x)
