func (s *Set) Len() int {
	return len(s.elements)
}
Len gets elements count
package util
type Set struct {
	elements []string
	index    map[string]struct{}
}
func NewSet() Set {
	return Set{
		elements: []string{},
		index:    map[string]struct{}{},
	}
}
func (s *Set) Add(element string) bool {
	if s.Exists(element) {
		return false
	}
	s.elements = append(s.elements, element)
	s.index[element] = struct{}{}
	return true
}
func (s *Set) Exists(element string) bool {
	_, ok := s.index[element]
	return ok
}
func (s *Set) Elements() []string {
	return s.elements
}