package main

/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */
// 1 <= m <= n <= len of list
func reverseBetween(head *ListNode, m int, n int) *ListNode {
	var pre *ListNode
	curr, next := head, head.Next
	for cnt := 1; next != nil; cnt++ {
		if cnt == m {
			beforeBeg, beg := pre, curr
			for ; cnt != n; cnt++ {
				curr.Next = pre
				pre = curr
				curr = next
				next = next.Next
			}
			if beforeBeg != nil { // before beg -> end
				beforeBeg.Next = curr
			} else {
				head = curr // head is end
			}
			curr.Next = pre // end -> pre
			beg.Next = next // beg -> after end
			break
		}
		pre = curr
		curr = next
		next = next.Next
	}
	return head
}

// func main() {
// 	n3 := ListNode{3, nil}
// 	n2 := ListNode{2, &n3}
// 	n1 := ListNode{1, &n2}
// 	n0 := ListNode{0, &n1}
// 	fmt.Println(list2str(reverseBetween(&n0, 3, 4)))
// }