已知p指向双向循环链表中的一个结点，其结点结构为data、prior、next三个域，写出算法change(p)，交换p所指向的结点和它的前缀结点的顺序。

#include<assert.h>
typedef struct SLnode
{
	int data;
	struct SLnode* prior;
	struct SLnode* next;
}SLnode,*SLnodelist;
//创建结点
SLnode* createhead(int data)
{
	SLnode* newnode = (SLnode*)malloc(sizeof(SLnode));
	newnode->data = data;
	newnode->next = newnode->prior = newnode;
	return newnode;
}
//初始化哨兵位
SLnode* ListInit()
{
	SLnode* newnode = createhead(0);
}
//尾插
void ListPushBank(SLnode* plist, int x)
{
	assert(plist);
	SLnode* ret = plist->prior;
	SLnode* newnode = createhead(x);
 
	ret->next = newnode;
	newnode->next = plist;
	newnode->prior = ret;
	plist->prior = newnode;
}
void printlist(SLnode* plist)
{
	SLnode* ret = plist->next;
	while (ret != plist)
	{
		printf("%d->", ret->data);
		ret = ret->next;
	}
	printf("NULL\n");
}
void change(SLnodelist p)
{
	SLnodelist q;
	q = p->prior;
 
	q->prior->next = p;
	p->prior = q->prior;
 
	q->next = p->next;
	p->next->prior = q;
 
	q->prior = p;
	p->next = q;
 
}
int main()
{
	int n, num;
	SLnode* plist = ListInit();
	printf("请输入链表的长度\n");
	scanf("%d", &n);
	printf("请输入元素\n");
	for (int i = 0; i < n; i++)
	{
		scanf("%d", &num);
		ListPushBank(plist,num);
	}
	printf("双链表的元素如下\n");
	printlist(plist);
	printf("请输入你要交换的的结点的值\n");
	SLnodelist q = (SLnodelist)malloc(sizeof(SLnode));
	scanf("%d", &q->data);
	SLnodelist p;
	p = plist->next;
	while (p != plist)
	{
		if (p->data == q->data)
		{
			change(p);
			break;
		}
		else
		{
			p = p->next;
		}
	}
	printf("交换后\n");
	printlist(plist);
	return 0;
}