package q25

type ListNode struct {
	Val  int
	Next *ListNode
}

func reverse(begin *ListNode, k int) (newHead, newTail *ListNode) {
	tail := begin
	for range k - 1 {
		if tail.Next == nil {
			return begin, tail // no enough nodes
		}
		tail = tail.Next
	}

	// Reverse
	cur := begin
	prev := tail.Next
	for range k - 1 {
		t := cur.Next
		cur.Next = prev
		prev, cur = cur, t
	}
	cur.Next = prev
	return cur, begin
}

func reverseKGroup(head *ListNode, k int) *ListNode {
	if k < 2 {
		return head
	}

	var ret, prevTail *ListNode
	p := head
	for p != nil {
		newHead, newTail := reverse(p, k)
		if ret == nil {
			ret = newHead
		}
		if prevTail != nil {
			prevTail.Next = newHead
		}
		prevTail = newTail
		p = newTail.Next
	}

	return ret
}

var _ = reverseKGroup