为什么会出现写入访问权限冲突。 tail 是 nullptr？

[Re:从零开始的代码生活]

题目要求将链表逆置，输入-1结束。

代码：

#define _CRT_SECURE_NO_DEPRECATE
#include <stdio.h>
#include <stdlib.h>

struct ListNode {
    int data;
    struct ListNode* next;
};

struct ListNode* createlist();
struct ListNode* reverse(struct ListNode* head);
void printlist(struct ListNode* head)
{
    struct ListNode* p = head;
    while (p) {
        printf("%d ", p->data);
        p = p->next;
    }
    printf("\n");
}

int main()
{
    struct ListNode* head;

    head = createlist();
    head = reverse(head);
    printlist(head);

    return 0;
}
struct ListNode* createlist()
{
    struct ListNode* head = NULL, * tail = NULL, * p = NULL;
    int data;
    scanf("%d", &data);
    while (data != -1)
    {
        p = (struct ListNode*)malloc(sizeof(struct ListNode));
        p->data = data;
        p->next = NULL;
        if (head == NULL)
            head = p;
        else
            tail->next = p;
        tail = p;
        scanf("%d", &data);
    }
    return head;
}
struct ListNode* reverse(struct ListNode* head)
{
    struct ListNode* ptr1, * ptr2, * heado = NULL, * tail = NULL;
    int a[100], index = 0;
    for (ptr1 = head; ptr1 != NULL; ptr1 = ptr1->next)
        a[index++] = ptr1->data;
    for (int i = index - 1; i >= 0; i--)
    {
        ptr2 = (struct ListNode*)malloc(sizeof(struct ListNode));
        ptr2->data = a[i];
        ptr2->next = NULL;
        if (heado = NULL)
            heado = ptr2;
        else
            tail->next = ptr2;
        tail = ptr2;
    }
    return heado;
}

[Simple-Soft]

if (heado = NULL)
heado = ptr2;
else
tail->next = ptr2;
这里错了，应该为if (heado == NULL)

[自信男孩]

struct ListNode* reverse(struct ListNode* head)
{
    struct ListNode* ptr1, * ptr2, * heado = NULL, * tail = NULL;
    int a[100], index = 0;
    for (ptr1 = head; ptr1 != NULL; ptr1 = ptr1->next)
        a[index++] = ptr1->data;
    for (int i = index - 1; i >= 0; i--)
    {
        ptr2 = (struct ListNode*)malloc(sizeof(struct ListNode));
        ptr2->data = a[i];
        ptr2->next = NULL;
        //if (heado = NULL)
        if (heado == NULL)
            heado = ptr2;
        else
            tail->next = ptr2;
        tail = ptr2;
    }
    return heado;
}

供参考～

=不是比较==是比较

[Re:从零开始的代码生活]

引用 1 楼 Simple-Soft的回复:

if (heado = NULL)
heado = ptr2;
else
tail->next = ptr2;
这里错了，应该为if (heado == NULL)

自己看了半个小时没看出来竟然犯了这么低级的错误，谢谢哈