linkedlist.go
func split(head *llNode) (left, right *llNode) {
	left = head
	if head == nil || head.next == nil {
		return
	}
	right = head
	sprinter := head
	prev := head
	for sprinter != nil && sprinter.next != nil {
		prev = right
		right = right.next
		sprinter = sprinter.next.next
	}
	prev.next = nil
	return
}
func split(head *llNode) (left, right *llNode) {
	left = head
	if head == nil || head.next == nil {
		return
	}
	right = head
	sprinter := head
	prev := head
	for sprinter != nil && sprinter.next != nil {
		prev = right
		right = right.next
		sprinter = sprinter.next.next
	}
	prev.next = nil
	return
}
// split breaks a list in half.