#include <iostream>
#include <string>

using namespace std;

struct Student {
    int id;
    string name;
    int age;
    Student* next;
};

Student* head = nullptr; // Define the head of the linked list

Student* createStudent(int id, const string& name, int age) {
    Student* s = new Student;
    s->id = id;
    s->name = name;
    s->age = age;
    s->next = nullptr;
    return s;
}

void addStudent(int id, const string& name, int age) { // Add student to the end
    Student* s = createStudent(id, name, age);

    if (!head) {
        head = s;
        return;
    }

    Student* p = head;
    while (p->next) {
        p = p->next;
    }
    p->next = s;
}

Student* findStudent(int id) {
    Student* p = head;
    while (p) {
        if (p->id == id)
            return p;
        p = p->next;
    }
    return nullptr;
}

bool updateStudent(int id, const string& newName, int newAge) {
    Student* s = findStudent(id);
    if (!s)
        return false;

    s->name = newName;
    s->age = newAge;
    return true;
}

bool deleteStudent(int id) {
    if (!head)
        return false;

    if (head->id == id) { // If head is to be deleted
        Student* tmp = head;
        head = head->next;
        delete tmp;
        return true;
    }

    Student* p = head;
    while (p->next && p->next->id != id) {
        p = p->next;
    }

    if (p->next) {
        Student* tmp = p->next;
        p->next = tmp->next;
        delete tmp;
        return true;
    }

    return false;
}

void showStudents() {
    if (!head) {
        cout << "No student info.\n";
        return;
    }

    Student* p = head;
    cout << "Student ID\tName\tAge\n";
    while (p) {
        cout << p->id << "\t" << p->name << "\t" << p->age << endl;
        p = p->next;
    }
}

void menu() {
    cout << "\n===== Student Mgmt System =====\n";
    cout << "1. Add student\n";
    cout << "2. Delete student\n";
    cout << "3. Modify student\n";
    cout << "4. Query student\n";
    cout << "5. Display all students\n";
    cout << "0. Exit\n";
    cout << "Please select: ";
}

int main() {
    int choice;

    do {
        menu();
        cin >> choice;

        int id, age;
        string name;

        switch (choice) {
        case 1:
            cout << "Student ID - Name - Age: ";
            cin >> id >> name >> age;
            addStudent(id, name, age);
            cout << "Added successfully.\n";
            break;

        case 2:
            cout << "Input student ID to delete: ";
            cin >> id;
            if (deleteStudent(id))
                cout << "Deleted.\n";
            else
                cout << "Student not found.\n";
            break;

        case 3:
            cout << "Enter new Student ID - Name - Age: ";
            cin >> id >> name >> age;
            if (updateStudent(id, name, age))
                cout << "Modified successfully.\n";
            else
                cout << "Student not found.\n";
            break;

        case 4: {
            cout << "Enter Student ID: ";
            cin >> id;
            Student* s = findStudent(id);
            if (s)
                cout << "Student ID: " << s->id << " Name: " << s->name << " Age: " << s->age << endl;
            else
                cout << "Student not found.\n";
            break;
        }

        case 5:
            showStudents();
            break;

        case 0:
            cout << "Exit.\n";
            break;

        default:
            cout << "Invalid option.\n";
        }

    } while (choice != 0);

    return 0;
}