#include <iostream>

using namespace std;


struct node {
    int val;
    node * next;
};

struct SLL {
    node * head;

    void init() {
        head = NULL;
    }

    void insertFirst(int val) {
        node * new_node = new node;
        new_node->val = val;

        if(head == NULL) {
            head = new_node;
        }
        else {
            new_node->next = head;
            head = new_node;
        }
    }

    void insertLast(int val) {
        node * new_node = new node;
        new_node->val = val;

        if(head == NULL) {
            head = new_node;
        }
        else if(head->next == NULL) {
            head->next = new_node;
            new_node->next = NULL;
        }
        else {
            node * tmp = head;
            while(tmp->next != NULL) {
                tmp = tmp->next;
            }
            tmp->next = new_node;
            new_node->next = NULL;
        }
    }

    void deleteFirst() {
        if(head != NULL) {
            if(head->next == NULL) {
                delete head;
                head = NULL;
            }
            else {
                node * tmp = head;
                head = head->next;
                delete tmp;
            }
        }
    }
    void deleteLast() {
        if(head != NULL) {
            if(head -> next == NULL) {
                delete head;
                head =  NULL;
            }
            else {
                node * tmp = head;
                while(tmp -> next -> next != NULL) {
                    tmp = tmp->next;
                }
                node * tmp2 = tmp->next;
                delete tmp2;
                tmp->next = NULL;
            }
        }
    }
    void print() {
        node * tmp = head;
        while(tmp != NULL) {
            cout << tmp->val << " --> ";
            tmp = tmp->next;
        }
        cout << endl;
    }
};
int main()
{
    SLL sll;
    sll.init();

    sll.insertFirst(1);
    sll.insertFirst(2);
    sll.insertFirst(3);
    sll.print();

    sll.insertLast(4);
    sll.insertLast(5);
    sll.insertLast(6);

    sll.print();


    sll.deleteFirst();
    sll.deleteLast();
    sll.print();




    return 0;

}