C++学生信息系统 链表运行中止

#include <iostream>
#include <string>
#include <stdio.h>
using namespace std;

typedef struct Student {
    string number;
    string name;
}st;
typedef struct node {
    st data;
    struct node* next;
}Node,*Link;

void input(Link l) {
    cout << "please enter the student's number: ";
    cin >> l->data.number;
    cout << "please enter the student's name : ";
    cin >> l->data.name;
    l->next = NULL; //!!重要 //#include <stdio.h>
}
bool insert(Link head){
    Link p, q;
    Link node;
    node = (Link)malloc(sizeof(Node));
    input(node);
    q = head;
    p = head->next;
    if (head->next == NULL)
        head->next = node;
    else{
        while (p != NULL) {
            if (node->data.number < p->data.number /**/ && node->data.number>q->data.number) {
                q->next = node;
                node->next = p;
                return true;
            }
            else {
                q = p;
                p = p->next;
            }
        }
        q->next = node;//q而不是p,p已经是NULL了！
    }
    return true;
}
void show(Link head,string numnam) {
    Link p;
    p = head->next;
    while (p != NULL) {
        if (p->data.name == numnam || p->data.number == numnam) {
            cout << "找到该学生相关信息如下：\n";
            cout << "name:" << p->data.name << endl
                << "number:" << p->data.number << endl;
            break;
        }
        else {
            p = p->next;
        }
    }
    cout << "未找到该学生！\n";
}
void showall(Link head) {
    Link p;
    p = head->next;
    cout << "找到所有学生相关信息如下：\n";
    while (p != NULL) {
            cout << "name:" << p->data.name << endl
                << "number:" << p->data.number << endl;
            p = p->next;
    }
    cout << "所有学生信息显示完毕！\n";
}
void modifyname(Link head, string numnam) {
    Link p;
    p = head->next;
    while (p != NULL) {
        if (p->data.name == numnam || p->data.number == numnam) {
            cout << "请输入修改后的学生姓名：\n";
            cin >> p->data.name;
        }
        else {
            p = p->next;
        }
    }
    cout << "修改失败，未找到该学生！\n";
}
int main() {
    Link head;
    head = (Link)malloc(sizeof(Node));
    head->next = NULL;
    int n;
    cout << "您需要输入多少学生信息？：";
    cin >> n;
    for (int i = 0; i < n; i++)
        cout << insert(head);
    cout << "所有学生信息如下：\n";
    showall(head);
    return 0;
}

求大lao告知我的链表代码哪里出错了
求球啦！九救 子亥 子 吧

因为你head没有初始化，第一次insert直接插到head后面没有问题，但是后面的insert的时候又用到head，固然下面那个比较就有可能不成立了。因为head没有初始化，里面的值未知。
if (node->data.number < p->data.number /**/ && node->data.number>q->data.number)

cout << insert(head);
为啥cout输出insert的返回值呢？

void insert(Link head){
Link p;
Link node;
node = (Link)malloc(sizeof(Node));
input(node);
p = head;
while(p->next!=null)
{
if(node->data.number < p->next->data.number)
{
node->next=p->next;
break;
}
p=p->next;
}
p->next=node;
}

虽然结题了，还是发下修改的代码，用string类也是可以的，不管是学号还是姓名，因为string字符串用来查找、比较很方便，供参考：

#include <iostream>
#include <string>
#include <stdio.h>
using namespace std;

typedef struct Student {
    string number;
    string name;
}st;
typedef struct node {
    st     data;
    struct node* next;
}Node,*Link;
void input(Link l) {
    l->data.number.reserve(16);
    l->data.name.reserve(20);
    cout << "please enter the student's number: ";
    cin >>l->data.number;
    cout << "please enter the student's name : ";
    cin >>l->data.name;
    l->next = NULL; //!!重要 //#include <stdio.h>
}
bool insert_m(Link head){
    Link p, q;
    Link node;
    node = (Link)malloc(sizeof(Node));
    input(node);
    q = head;
    p = head->next;
    if (head->next == NULL)
        head->next = node;
    else{
        while (p != NULL) {
            if (node->data.number < p->data.number){// && node->data.number>q->data.number) {
                q->next = node;
                node->next = p;
                return true;
            }
            else {
                q = p;
                p = p->next;
            }
        }
        q->next = node;//q而不是p,p已经是NULL了！
    }
    return true;
}
void show(Link head,string numnam) {
    Link p;
    p = head->next;
    while (p != NULL) {
        if (p->data.name == numnam || p->data.number == numnam) {
            cout << "找到该学生相关信息如下：\n";
            cout << "name:" << p->data.name << endl
                << "number:" << p->data.number << endl;
            break;
        }
        else {
            p = p->next;
        }
    }
    cout << "未找到该学生！\n";
}
void showall(Link head) {
    Link p;
    p = head->next;
    cout << "找到所有学生相关信息如下：\n";
    while (p != NULL) {
            cout << "name:" << p->data.name << endl
                << "number:" << p->data.number << endl;
            p = p->next;
    }
    cout << "所有学生信息显示完毕！\n";
}
void modifyname(Link head, string numnam) {
    Link p;
    p = head->next;
    while (p != NULL) {
        if (p->data.name == numnam || p->data.number == numnam) {
            cout << "请输入修改后的学生姓名：\n";
            cin >> p->data.name;
        }
        else {
            p = p->next;
        }
    }
    cout << "修改失败，未找到该学生！\n";
}
int main() {
    Link head;
    head = (Link)malloc(sizeof(Node));
    head->next = NULL;
    int n;
    cout << "您需要输入多少学生信息？：";
    cin >> n;
    for (int i = 0; i < n; i++)
         insert_m(head); //cout << insert(head);
    cout << "所有学生信息如下：\n";
    showall(head);

    show(head,"aaaa");

    system("pause");
    return 0;
}