Update 92_Reverse_Linked_List_II.java

Author: Sean Prashad

In-place Reveral of a Linked List/92_Reverse_Linked_List_II.java

@@ -1,21 +1,26 @@
 class Solution {
     public ListNode reverseBetween(ListNode head, int m, int n) {
+        if (head == null) {
+            return null;
+        }
+
         ListNode dummy = new ListNode(-1);
         dummy.next = head;
 
-        ListNode begin = dummy;
+        ListNode prev = dummy;
 
         for (int i = 0; i < m - 1; i++) {
-            begin = begin.next;
+            prev = prev.next;
         }
 
-        ListNode start = begin.next, then = start.next;
+        ListNode start = prev.next;
 
         for (int i = 0; i < n - m; i++) {
+            ListNode then = start.next;
+
             start.next = then.next;
-            then.next = begin.next;
-            begin.next = then;
-            then = start.next;
+            then.next = prev.next;
+            prev.next = then;
         }
 
         return dummy.next;