/* stack.c
 *
 * Example for CS 313  (compare to Pascal version stack.p!)
 *
 * Daniel Scharstein
 *
 * to compile, use  gcc -W -Wall -o stack stack.c
 */

#include <stdio.h>  /* for printf */
#include <stdlib.h> /* for malloc, free */

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

typedef struct elt *intlist;

intlist mystack;
/* global variable - generally considered bad style.
 * Don't use in your homework!
 */

void push(int n) /* push n on stack */
{
    intlist e = (intlist)malloc(sizeof(struct elt)); /* allocates new memory */
    e->val = n;
    e->next = mystack;
    mystack = e;
}

int empty() /* return whether is stack empty */
{ 
    return (mystack == NULL);
}

int pop() /* pop a value off stack and return it */
{
    if (empty())
	return 0;
    else {
	intlist e = mystack;
	int v = e->val;
	mystack = mystack->next;
	free(e);  /* frees memory that was used to hold first elt */
	return v;
    }
}
   
void printstack() /* print current contents of stack */
{
    intlist e;
    printf("Stack: ");
    for (e = mystack; e != NULL; e = e->next)
	printf("%d ", e->val);
    printf("\n");
}


/* main program: */
int main()
{
    int i;
    mystack = NULL;
    for (i=1; i<=5; i++) {
	printf("Push %d\n", i);
	push(i);
    }
    printf("Pop %d\n", pop());
    printstack();
    printf("Pop %d\n", pop());
    printf("Push %d\n", 6); push(6);
    while (! empty())
	printf("Pop %d\n", pop());
    printf("top %d\n", mystack->val);
    return 0;
}