typedef struct stack stack;
typedef struct node node;

struct node {
	char sequence;
	node *next;
};

struct stack {
	node *top;
};

stack *create_stack() {
	stack *new_stack = (stack*) malloc(sizeof(stack));
	new_stack -> top = NULL;
	return new_stack;
}

void push(stack *stack, char sequence) {
	node *new_top = (node*) malloc(sizeof(node));
	new_top -> sequence = sequence;
	new_top -> next = stack -> top;
	stack -> top = new_top;
	return;
}

int pop(stack *stack) {
	if(stack -> top == NULL) {
		//printf("Stack is underflow.\n");
		return -1; // error code
	}
	else {
		//printf("%s\n", stack -> top -> sequence);
		stack -> top = stack -> top -> next;
		return 0; // success code
	}
}