#include <stdio.h>
#include <stdlib.h>

typedef struct node node;

struct node {
    node *next;
    node *prev;
    int key;
};

void addTail(node *list, int key);
void list_show(node *list);

int main() {

    node *list = (node*) malloc(sizeof(node));                  //old node *new_list_node()  but structure name is list
    list->prev = NULL;                                          //old node *new_list_node()
    list->next = NULL;                                          //old node *new_list_node()
    list->key = NULL;                                           //old node *new_list_node()

    addTail(list, 10);
    addTail(list, 15);
    addTail(list, 20);
    list_show(list);
}

void addTail(node *list, int key) {
    if (list->key == NULL) {
        list->key = key;
    } else {
        while(list->next != NULL) {
            list = list->next;
        }

        node *new_node = (node*) malloc(sizeof(node));
        new_node->next = NULL;
        new_node->prev = list;
        new_node->key = key;
        list->next = new_node;
    }
}

void list_show(node *list) {
    while (list != NULL) {
        printf("%d ", list->key);
        list = list->next;
    }
}