/* The stack pointer always points to the first free position on the stack. This is the common way to implement  the stack. */

#include <stdlib.h>

struct Node {
  int element;
  Node* next;
};

Node* stackPointer = NULL;

void push(int element) {
  Node* newNode = (Node*)malloc(sizeof(Node));
  newNode->element = element;
  newNode->next = stackPointer;
  stackPointer = newNode;
}

bool isEmpty() {
  return stackPointer == NULL;
}

int pop() {
  if (!isEmpty()) {
    Node* tmp = stackPointer;
    int element = stackPointer->element;
    stackPointer = stackPointer->next;
    free(tmp);
    return element;
  } 
}