Linked List C++. Lỗi khi insert vào vị trí K. Ai giúp với ạ

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#include "iostream"
#include "string"
#include "conio.h"
#include "cstdlib"
using namespace std;
struct ThongTin
{
    string name;
    string address;
    int old;
};
struct SinhVien
{
    ThongTin imformation;
    SinhVien *next;
};

typedef SinhVien* List;

void initializer(List &L)
{
    L = NULL;
}
int length(List L)
{
    SinhVien* Q = L;
    int dem = 0;
    while(Q!=NULL)
    {
        dem ++;
        Q = Q->next;
    }
    return dem;
}
bool isEmpty(List L)
{
    if (L == NULL)
    {
        return true;
    } else return false;
}
void Nhap(ThongTin &Ithongtin)
{
    cin.ignore();
    cout<<" Nhap Ten : "; getline(cin,Ithongtin.name);
    cout<<" Tuoi : "; cin>>Ithongtin.old;
    cin.ignore();
    cout<<" Dia Chi : "; getline(cin,Ithongtin.address);
}
SinhVien* MakeNode(SinhVien *P,ThongTin thongtin)
{
    P = new SinhVien;
    P->next = NULL;
    P->imformation.name = thongtin.name;
    P->imformation.old = thongtin.old;
    P->imformation.address = thongtin.address;
    return P;
}
void insertFirst(List &L)
{
    SinhVien *P;
    ThongTin Fthongtin;
    Nhap(Fthongtin);
    P = MakeNode(P,Fthongtin);
    P->next = L;
    L = P;
}
void insertK(List &L,int k){
    if(k<1 || k > length(L)+1)
    {
        cout<<"Vi tri khong hop le " <<endl;
    }
    else if(k == 1)
    {
        insertFirst(L);
    }
    else
    {
        ThongTin Kthongtin;
        SinhVien *Q = L;
        int vitri = 0;
        SinhVien*P ;
        P = MakeNode(P,Kthongtin);
        while(Q!= NULL && vitri != k-1)
        {
            vitri++;
            Q = Q->next;
        }
        P->next = Q->next;
        Q->next = P;
    }
}
void Xuat(List L)
{
    SinhVien *Q =L;
    while(Q!=NULL){
        cout<<"Name : "<<Q->imformation.name<<endl;
        cout<<"Old : "<<Q->imformation.old<<endl;
        cout<<"Address : "<<Q->imformation.address<<endl;
        Q = Q->next;
    }
}
void Menu(List L)
{
    int position;
    int choice;
while(1){
    cout<<"1 : Nhap vao dau Danh Sach "<<endl;
    cout<<"2 : Nhap vao vi tri cho truoc"<<endl;
    cout<<"3 : Xuat Danh Sach "<<endl;
    cout<<"4 : Check Empty List "<<endl;
    cout<<"5 : Ket Thuc "<<endl;
    cout<<" Ban chon so : "; cin>>choice;

    switch(choice)
        {
        case 1:
            insertFirst(L);
            break;
        case 2:
            cout<<"Import position you need add data "; cin>>position;
            cin.ignore();
            insertK(L,position);
            break;
        case 3:
            Xuat(L);
            break;
        case 4:
            if(isEmpty(L))
                cout<<"List is Empty"<<endl;
            else cout<<"List is'nt Empty"<<endl;
            break;
        case 5:
            exit(0);
        default :
            cout<<"Nhap so tu 1 den 4" <<endl;
            break;
        }
    }
}
int main()
{
    List L;
    initializer(L);
    Menu(L);
    return 0;
}