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

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

struct node *front;
struct node *back;

void add_to_front(int v);
void remove_from_back();
void print_list();

int main() {
   int value;
   while (1) {
      printf("Enter a number: ");
      int rtn = scanf("%d", &value);
      if (rtn != 1) 
         break;
      else
         add_to_front(value);
   }

   print_list();

   remove_from_back();
   print_list();

   remove_from_back();
   print_list();
 
   remove_from_back();
   print_list();

   return 0;
}

void add_to_front(int v)
{
   struct node *new_node;
   new_node = (struct node *) malloc(sizeof(struct node));
   if (new_node == NULL) {
      printf("Error: malloc failed in add_to_front\n");
      exit(EXIT_FAILURE);
   }
   new_node->value = v;

   if (front == NULL) { //special case, empty list
      front = back = new_node;
   } else {
      new_node->next = front;
      front->prev = new_node; //update prev pointer
      front = new_node;
   }
}
 
void remove_from_back()
{
   if (back == NULL) return; //list is empty
   if (back->prev == NULL) { //special case, one node
      free(back);
      front = back = NULL;
   } else {
      struct node *to_free = back;
      back = to_free->prev;
      to_free->prev = NULL;
      back->next = NULL;
      free(to_free);
   }
}

void print_list() 
{
   struct node *cur;
   for (cur = front; cur != NULL; cur = cur->next) {
      printf("%d ", cur->value);
   }
   printf("\n");
}