[Error] ld returned 1 exit status是什么原因，一会能运行，一会不行！


#include
#include

struct Node
{
    int value;
    struct Node *next; 
 };
 
void insertNode(struct Node **head, int value)
{
    struct Node *previous;
    struct Node *current;
    struct Node *new;
    
    current = *head;
    previous = NULL;
    
    while (current != NULL && current->value < value)
    {
        previous = current;
        current = current->next;
    }
    
    new = (struct Node *)malloc(sizeof(struct Node));
    if (new == NULL)
    {
        printf("内存分配失败！\n");
        exit(1);
     } 
     
     new->value = value;
     new->next = current;
     
     if (previous = NULL)
     {
         *head = new;
     }
     else
     {
         previous->next = new;
     }
     
}

void printNode(struct Node *head)
{
    struct Node *current;
    
    current = head;
    while (current != NULL)
    {
        printf("%d", current->value);
        current = current->next;
    }
    
    putchar('\n');
    
}
  
int main(void)
{
    struct Node *head = NULL;
    int input;
    
    while(1)
    {
        printf("请输入一个整数(输入-1表示结束)：");
        scanf("%d", &input);
        if (input == -1)
        {
            break;
         } 
         insertNode(&head, input);
         printNode(head);
    }
    
    return 0;
}

if (previous = NULL)
改成
if (previous == NULL)