数据结构链表，完全对应书上，但在vs2019上无法运行。

Question

1. #pragma once
   #ifndef _clist_
   #define _clist_
   
   struct node;
   typedef struct head* phead;
   typedef struct node* pnode;
   
   struct head
   {
   	int length;
   	pnode next;
   };
   struct node
   {
   	int data;
   	pnode next;
   };
   phead clistcreate();
   int getlength(phead ph);
   int isempty(phead ph);
   int clistinsert(phead ph, int pos, int val);
   void print(phead ph);
   
   
   #endif // !_clist_//头文件
   
   #include"标头.h"
   #include<stdio.h>
   #include<stdlib.h>
   
   int main()
   {
   	int m, n;
   	printf("请输入约瑟夫环的总人数！\n");
   	scanf_s("%d", &m);
   	printf("请输入需要踢出的人数！\n");
   	scanf_s("%d", &n);
   	phead ph = NULL;
   	ph = clistcreate();
   	if (ph == NULL)
   	{
   		printf("约瑟夫环创建失败！\n");
   		return 0;
   	}
   
   	for (int i = m; i > 0; i++)
   	{
   		clistinsert(ph, 0, i);
   	}
   	print(ph);
   	printf("被踢顺序：\n");
   	pnode node = ph->next;
   	while (node->next != node)
   	{
   		for (int i = 1; i < n - 1; i++)
   		{
   			node = node->next;
   		}
   		pnode ptmp = node->next;
   		if (ptmp == ph->next)
   		{
   			ph->next = ptmp->next;
   			node->next = ptmp -> next;
   			printf("%d", ptmp->data);
   			free(ptmp);
   			ph->length--;
   		}
   		else
   		{
   			node->next = ptmp->next;
   			printf("%d", ptmp->data);
   			free(ptmp);
   			ph->length--;
   		}
   		node = node->next;
   
   	}
   	node->next = node;
   	printf("\n");
   	printf("链表中最后留下的是：");
   	print(ph);
   	system("pause");
   	return 0;
   
   }//函数
   
   #include"标头.h"
   #include <stdio.h>
   #include<stdlib.h>
   
   phead clistcreate()
   {
   	phead ph = (phead)malloc(sizeof(struct head));
   	if (ph == NULL)
   		printf("头节点分配失败！");
   	ph->length = 0;
   	ph->next = NULL;
   	return ph;
   }
   int isempty(phead ph)
   {
   	if (ph == NULL)
   		printf("传入的链表有误！");
   	if (ph->length == 0)
   		return 1;
   	else
   		return 0;
   }
   
   int clistinsert(phead ph, int pos, int val)
   {
   	if (ph == NULL || pos<0 || pos>ph->length)
   	{
   		printf("插入元素时，元素传入有误！");
   	}
   	pnode pval;
   	pval = (pnode)malloc(sizeof(node));
   	pval = NULL;
   	pval->data = val;
   	if (isempty)
   	{
   		ph->next = pval;
   		pval->next = pval;
   	}
   	else
   	{
   		pnode prear = ph->next;
   		if (pos == 0)
   		{
   			while (prear->next != ph->next)
   			{
   				prear = prear->next;
   			}
   			pval->next = ph->next;
   			ph->next = pval;
   			prear->next = pval;
   		}
   		else
   		{
   			pnode pcur = ph->next;
   			for (int i = 1; i < pos; i++)
   			{
   				pcur = pcur->next;
   			}
   			pval->next = pcur->next;
   			pcur->next = pval;
   		}
   	}
   	ph->length++;
   	return 1;
   }
   
   void print(phead ph)
   {
   	if (ph == NULL || ph->length == 0)
   	{
   		printf("参数传入时有误！");
   	}
   	pnode ptmp = ph->next;
   
   	for (int i = 0; i < ph->length; i++)
   	{
   		printf("%d", ptmp->data);
   		ptmp = ptmp->next;
   	}
   	printf("\n");
   }
   

vs2019显示问题出在插入函数中的pval->data=val;

引发了异常: 写入访问权限冲突。
**pval** 是 nullptr。

Answer 1

书也是人写的，人非圣贤，孰能无过。

自己改代码就好，这就是插入函数运行 pval->data=val; 的时候 pval是空指针，所欲pval->xxxx会报错，引起程序崩溃。

	
	pnode pval;
	pval = (pnode)malloc(sizeof(node));
	pval = NULL;     //这一句将pavl设置成空指针了，删了此句就行
	pval->data = val;

如上面注释的一样，把 pval = NULL删了就行。