lalki povara

#include <iostream>
#include <cstdio>
#include <stdexcept>

template <typename T>
class List {
public:
    List();
    void push_back(const T &data);
    void pop_back();
    unsigned int size();
    T back();
    T front();

private:
    template <typename X>
    struct Node {
        X data;
        Node *next;
        Node(const X &data, const Node *next) : data(data), next(next) {}
        Node(const X &data) : data(data), next(NULL) {}
    };
    Node <T> *head;
    Node <T> *tail;
    unsigned int length;
};

template <typename T>
List <T>::List(): head(NULL), tail(NULL), length(0) { }

template <typename T>
void List <T>::push_back(const T &data) {
    if (!head) {
        head = new Node <T>(data);
        tail = head;
    }
    else {
        tail->next = new Node<T>(data);
        tail = tail -> next;
    }
    length += 1;
}

template <typename T>
unsigned int List <T>::size() {
    return length;
}

template <typename T>
void List <T>::pop_back() {
    if (!tail) {
        throw std::out_of_range("Can't get data from empty list");
    }
    if (head == tail) {
        delete head;
        head = NULL;
        tail = NULL;
    } else {
        Node <T> *penult = head;
        while (penult->next != tail) {
            penult = penult -> next;
        }
        tail = NULL;
        tail = new Node <T>(penult -> data);
    }
    length--;
    return;
}

template <typename T>
T List <T>::back() {
    if (!tail) {
        throw std::out_of_range("Can't get data from empty list");
    } else {
        return tail -> data;
    }
}

template <typename T>
T List <T>::front() {
    if (!tail) {
        throw std::out_of_range("Can't get data from empty list");
    } else {
        return head -> data;
    }
}

int main() {
    List <int> a;
    a.push_back(2);
    a.push_back(3);
    std::printf("%d", a.back());
    std::printf("%d", a.front());
    a.pop_back();
    std::printf("%d", a.back());
    return 0;
}