Односвязный список (базовые функции)

#include "pch.h"
#include <iostream>

struct Node
{
    int data;
    Node* next;
};

class MyList {
    Node* head;
    Node* tail = new Node;
public:
    bool is_empty()
    {
        return head == NULL;
    }

    int pop()
    {
        int data;
        Node* p;
        if (tail)
        {
            data = tail->data;
            p = previous(tail);
            p->next = NULL;
            delete tail;
            tail = p;
        }
        return data;
    }

    void pop(int &data)
    {
        Node* p;
        if (!is_empty())
        {
            data = tail->data;
            p = tail->next;
            delete tail;
            tail = p;
        }
    }

    void push(int data)
    {
        Node* p = new Node;
        p->data = data;
        p->next = NULL;
        if (is_empty()) {
            head = tail = p;
        }
        else {
            tail->next = p;
            tail = p;
        }
    }

    void show_list()
    {
        Node* p;
        p = head;
        while (p)
        {
            std::cout << p->data << " ";
            p = p->next;
        }
        std::cout << std::endl;
    }

    Node* previous(Node* p) {
        Node* pr, *q;
        q = head;
        pr = NULL;
        while (q != p && q) {
            pr = q;
            q = q->next;
        }
        if (q == p) return pr;
        return NULL;
    }

    void insert_before(Node* q, int x)
    {
        Node* p, *r;
        p = new Node;
        r = previous(q);
        if (q == tail)
            push(x);
        else
            if (r == NULL)
            {
                std::cout << " sorry, impossible insert element " << std::endl;
                exit(-1);
            }
            else
            {
                p->data = x;
                p->next = r->next;
                r->next = p;
            }
    }

    void insert_after(Node* &q, int x)
    {
        Node* p;
        p = new Node;
        p->data = x;
        p->next = q->next;
        q->next = p;
    }

    Node* find_address(int x)
    {
        Node* p = head;
        while (p && (p->data) != x)
            p = p->next;
        return p;
    }

    void delete_element(Node* p)
    {
        Node* r; int x;
        if (p == tail)
            pop(x);

        else
        {
            r = previous(p);
            r->next = p->next;
            delete p;
        }
    }
};

int main()
{
    MyList list;
    for (int i = 0; i < 150; i++) {
        list.push(i);
    }
    list.show_list();
    list.pop();
    list.show_list();
    auto a = list.find_address(20);
    list.insert_after(a, 505);
    list.show_list();
    list.insert_before(a, 404);
    list.show_list();
    list.delete_element(a);
    list.show_list();
}