package tools

// 单链表节点的结构
type ListNode struct {
	val  int
	next *ListNode
}

func NewListNode(x int) *ListNode {
	return &ListNode{
		val: x,
	}
}
func ReverseList(head *ListNode) *ListNode {
	if head.next == nil {
		return head
	}
	last := ReverseList(head.next)
	head.next.next = head
	head.next = nil
	return last
}

var successor *ListNode // 后驱节点
// 将链表的前 n 个节点反转（n <= 链表长度）
func ReverseListN(head *ListNode, n int) *ListNode {
	if n == 1 {
		// 记录第 n + 1 个节点
		successor = head.next
		return head
	}
	// 以 head.next 为起点，需要反转前 n - 1 个节点
	last := ReverseListN(head.next, n-1)
	head.next.next = head
	// 让反转之后的 head 节点和后面的节点连起来
	head.next = successor
	return last
}

func ReverseBetween(head *ListNode, m int, n int) *ListNode {
	if m == 1 {
		return ReverseListN(head, m)
	}
	// 前进到反转的起点触发 base case
	head.next = ReverseBetween(head.next, m-1, n-1)
	return head
}