在链表指定位置插入结点

#include <stdio.h>
#include <stdlib.h>
#include <iostream>
using namespace std;
struct student
{
    int  num;
    struct student* next;
};
//从键盘读入数据创建链表，新结点插入到尾部
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;  //首先置链表为空链表
    while (p1->num != -1)    //num为-1，意味着用户输入结束
    {
        n = n + 1;
        if (n == 1)            //创建第一个结点
            head = p1;
        else
            p2->next = p1;
        p2 = p1;            //p2始终指向最后一个结点（即尾指针）
        p1 = (struct student*)malloc(sizeof(struct student)); //p1指向新结点
        scanf("%d", &p1->num);
    }
    p2->next = NULL;  //最后一个结点的next赋值为NULL
    return head;
}
//输出链表中的信息（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");
}
//在链表中第index处插入s指针所指向的结点。index从1开始。
//由于可能插在第一个结点，所以函数返回头指针给主调函数
//head 是不带头结点的链表
struct student* insertNode(struct student* head, struct student* s, int index)
{
    if (!head && index != 1)return head;//处理空链表
    if (index == 1)//处理插在头
    {
        s->next = head;
        return s;
    }
    else
    {
        struct student* p = head,*q;
        int i = 1;
        while (p->next)//正常插入
        {
            if (i == index - 1)
            {
                s->next = p->next;
                p->next = s;
                break;
            }
            i++;
            p = p->next;
        }
        if (!p->next&&i==index-1)//插入到尾
        {
            p->next = s;
            s->next = NULL;
        }
        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;
}