understanding linked list c++ code example

Example 1: linkedlist implementation in c++

#include <iostream>

using namespace std;

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

class linked_list
{
private:
    node *head,*tail;
public:
    linked_list()
    {
        head = NULL;
        tail = NULL;
    }

    void add_node(int n)
    {
        node *tmp = new node;
        tmp->data = n;
        tmp->next = NULL;

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

int main()
{
    linked_list a;
    a.add_node(1);
    a.add_node(2);
    return 0;
}

Example 2: linked list in c++ stl

#include <bits/stdc++.h>
#include <iostream>
#include <list>
#include <iterator>

#define ll long long

using namespace std;

//function to print all the elements of the linked list
void showList(list <int> l){
	list <int> :: iterator it; //create an iterator according to the data structure
	for(it = l.begin(); it != l.end(); it++){
		cout<<*it<<" ";
	}
	
}	


int main(){
	
	list <int> l1;
	list <int> l2;
	
	for(int i=0; i<10; i++){
		l1.push_back(i*2); //fill list 1 with multiples of 2
		l2.push_back(i*3); //fill list 2 with multiples of 3
	}
	
	cout<<"content of list 1 is "<<endl;
	showList(l1);
	cout<<endl;
	
	cout<<"content of list 2 is "<<endl;
	showList(l2);
	cout<<endl;
	
	//reverse the first list
	l1.reverse();
	showList(l1);
	cout<<endl;
	
	//sort the first list
	l1.sort();
	showList(l1);
	cout<<endl;
	
	//removing an element from both sides
	l2.pop_front();
	l2.pop_back();
	
	//adding an element from both sides
	l2.push_back(10);
	l2.push_front(20);
	
	
    return 0;
}