Solution.java

public class Solution
{
    public ListNode reverseBetween(ListNode head, int m, int n)
    {
        if (head == null || m == n)
        {
            return head;
        }
        int i = 1;
        ListNode prev = null;
        ListNode start = head;
        ListNode end = head;
        while (i < n)
        {
            prev = i < m ? start : prev;
            start = i < m ? start.next : start;
            end = end.next;
            i++;
        }
        ListNode tail = end.next;
        end.next = null;
        reverse(start);
        if (prev != null)
        {
            prev.next = end;
        }
        start.next = tail;
        return m > 1 ? head : end;
    }

    private ListNode reverse(ListNode node)
    {
        if (node == null || node.next == null)
        {
            return node;
        }
        ListNode nxt = node.next;
        node.next = null;
        ListNode reverse = reverse(nxt);
        nxt.next = node;
        return reverse;
    }
}