#include <stdio.h>    // for NULL

#include <stdlib.h>   // for malloc(), free()

#include <assert.h>   // for assert()



#include "ll.h"



Node *Push(Node *head, void *element) {

  Node *newnode;



  // allocate the new node

  newnode = (Node *) malloc(sizeof(Node));

  assert(newnode != NULL);



  // initialize the new node

  newnode->element = element;

  newnode->next = head;



  return newnode;

}



void *Pop(Node *old_head, Node **new_head) {

  void *tmp;



  // Corner case: trying to Pop from an empty list.

  // Indicate failure by returning NULL.

  if (old_head == NULL)

    return NULL;



  // There's something in the list; let's save

  // what we need to return in a temporary variable.

  tmp = old_head->element;



  // Let's return (through the return parameter)

  // the new head of the list.

  *new_head = old_head->next;



  // Free up the old head.  Before doing that, we'll

  // defensively set its fields to NULL.  Why?

  // if there is some kind of pointer bug elsewhere,

  // this increases the chance it will manifest as

  // a (fail-stop) segmentation fault.

  old_head->element = NULL;

  old_head->next = NULL;

  free(old_head);



  // Return the payload we previously stashed in tmp.

  return tmp;

}