DLL

#include <iostream>

using namespace std;

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

class DLL{
    private:
    Node* root{};
    Node* tail;
    void clear_helper(Node* &curr);
    public:
    void clear();
    void add(int zahl);
    void push_front(int zahl);
    void push_back(int zahl);
    int pop_front();
    int pop_back();
    DLL(){};
    ~DLL(){ clear(); };
};


void DLL::clear_helper(Node* &curr) { //CLEAR_HELPER
    if (curr -> next != nullptr) {
        clear_helper(curr -> next);
    } else {
        delete curr;
        curr = nullptr;
    }
}

void DLL::clear() { //CLEAR FUNKTION
    if (root != nullptr) {
        clear_helper(root);
        delete tail;
        tail = nullptr;
    }
}


void DLL::push_front(int zahl){
    if (root == nullptr) {
        root = new Node;
        root -> data = zahl;
        tail = root;
    } else {
        Node* tmp = new Node;
        tmp -> data = zahl;
        tmp -> next = root;
        root -> prev = tmp;
        root = tmp;
    }
}

void DLL::push_back(int zahl) {
    if (root == nullptr) {
        root = new Node;
        root -> data = zahl;
        tail = root;
    } else {
        Node *tmp = new Node;
        tmp -> data = zahl;
        tmp -> prev = tail;
        tail -> next = tmp;
        tail = tmp;
    }
}

int DLL::pop_front() {
    if (root == nullptr) {
        throw logic_error("Liste leer");
    } else {
        Node* tmp = root;
        int ret = tmp -> data;
        root = root -> next;

        if (root == nullptr) {          //if last element was popped
            tail = nullptr;
        } else {
            root -> prev = nullptr;     //prev of first element has to be NULL
        }
        delete tmp;

        return ret;
    }
}

int DLL::pop_back() {
    if (root == nullptr) {
        throw logic_error("List already empty!");
    } else {
        int ret = tail -> data;

        if (root != nullptr && tail == root && root -> next == nullptr) {   //only one element in list
            delete tail;
            root = nullptr;
            tail = nullptr;
        } else {
            Node* tmp = tail;
            tail -> prev -> next = nullptr;
            tail = tail -> prev;
            delete tmp;
        }

        return ret;
    }
}

void DLL::add(int zahl) {
    if(root==nullptr){
        root = new Node;
        root->data = zahl;
        tail = root;
    }
    else{
        Node *curr = root;

        while (curr != nullptr && curr -> data > zahl) {
            curr = curr -> next;
        }

        if (curr -> prev == nullptr) {
            Node *tmp = new Node;
            tmp -> data = zahl;
            root = tmp;
            tmp -> next = curr;
            curr -> prev = tmp;
        } else if (curr == nullptr){
            Node *tmp = new Node;
            tmp -> data = zahl;
            tail -> next = tmp;
            tmp -> prev = tail;
        } else {
            Node *tmp = new Node;
            tmp -> data = zahl;

            tmp -> prev = curr -> prev;
            tmp -> next = curr;
            curr -> prev -> next = tmp;
            curr -> next -> prev = tmp;
        }
    }
}

int main(){
    DLL y;
    y.push_back(2);
    y.push_back(4);
    y.pop_back();
    y.pop_front();
}