type stack []rune

func (s *stack) push(b rune) {
	*s = append(*s, b)
}

func (s *stack) pop() (rune, bool) {
	if len(*s) > 0 {
		res := (*s)[len(*s)-1]
		*s = (*s)[:len(*s)-1]
		return res, true
	}
	return 0, false
}