DHU programming exercise

2 Insert a node at the specified position in the linked list

#include <stdio.h>
#include <stdlib.h>
struct student
{
    
	int  num;
	struct student  *next;
};
// Read data from the keyboard to create a linked list , Insert new node to tail 
struct student *createByTail()
{
    
	struct student *head;
	struct student *p1, *p2;
	int n;
	n = 0;
	p1 = p2 = (struct student*)malloc(sizeof(struct student));
	scanf("%d", &p1->num);
	head = NULL;  // First set the linked list as an empty linked list 
	while (p1->num != -1)    //num by -1, It means that the user input ends 
	{
    
		n = n + 1;
		if (n == 1)            // Create the first node 
			head = p1;
		else
			p2->next = p1;
		p2 = p1;            //p2 Always point to the last node （ Tail pointer ）
		p1 = (struct student*)malloc(sizeof(struct student)); //p1 Point to the new node 
		scanf("%d", &p1->num);
	}
	p2->next = NULL;  // Of the last node next The assignment is NULL
	return head;
}
// Output the information in the linked list （num）
void  displayLink(struct student *head)
{
    
	struct student *p;
	p = head;
	printf("head-->");
	while (p != NULL)
	{
    
		printf("%d-->", p->num);
		p = p->next;
	}
	printf("tail\n");
}
// In the list No index Insert s The node pointed to by the pointer .index from 1 Start .
// Because it may be inserted at the first node , So the function returns the header pointer to the calling function 
struct student *insertNode(struct student *head, struct student *s, int index)
{
    
	// Calculate the number of nodes in the original linked list , For later judgment index Whether the subscript paves the way normally 
	struct student *p;
	p = head;
	int n=0;
	while (p)
	{
    
		n++;
		p = p->next;
	}
	// Insert node , Consider the special case of the first position insertion 
	struct student *q;
	q = head;
	if (index <= n + 1)// Guarantee index Feasibility of subscript 
	{
    
		if (index == 1)
		{
    
			s->next = q;
			head = s;
		}
		else if (index!=0) {
     //Warning: here index Subscripts must be judged carefully ！！
			for (int i = 2; i < index; i++)// Guarantee q Point to the precursor node where the node is inserted 
			{
    
				q = q->next;
			}
			s->next = q->next;
			q->next = s;

		}
	}
	return head;
}
int main()
{
    
	struct student *head;
	int index, data;
	head = createByTail();
	while (scanf("%d%d", &index, &data) != -1)
	{
    
		struct student * s = (struct student *) malloc(sizeof(struct student));
		s->num = data;
		head = insertNode(head, s, index);
		displayLink(head);
	}
	return 0;
}