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

typedef int Item;

typedef struct node {
    Item data;
    struct node *next;
} StackNode;

// by putting top inside struct we get single pointer to hodl that does not change when top changes 
typedef struct {
    StackNode* top;
} StackType;

StackType* createStack()
{
    StackType* s = malloc(sizeof(StackType));
    return s;
}

void push(StackType* s, Item i)
{
    StackNode* newNode = malloc(sizeof(StackNode));
    newNode->data = i;
    newNode->next = s->top;
    s->top = newNode;
}

Item pop(StackType* s)
{
    StackNode* oldTop;
    if (s->top==NULL)
    {
	Item i;
	return i;
    }
    oldTop = s->top;
    s->top = oldTop->next;
    Item d = oldTop->data;
    free(oldTop);
    return d;
}

int main(void)
{
    StackType * s1;
    s1 = createStack();
    push(s1, 5);
    push(s1, 10);
    printf("%d\n", pop(s1));
    printf("%d\n", pop(s1));
}