queue.go
func (queue *queue) PeekBack() interface{} {
	if queue.size == 0 {
		return nil
	}

	return queue.elements[(queue.end-1)%int64(queue.cap)]
}
func (queue *queue) PeekBack() interface{} {
	if queue.size == 0 {
		return nil
	}

	return queue.elements[(queue.end-1)%int64(queue.cap)]
}
// PeekBack returns the element at the back of the queue.