Update Remove Nth Node From End of List.py

avoid use of prev, minimize time complexity

Author: resorcap

Remove Nth Node From End of List.py

@@ -1,11 +1,12 @@
 class Solution:
     def removeNthFromEnd(self, head, n):
-        fast, slow, prev = head, head, None
-        while n > 0:
-            fast, n = fast.next, n - 1
-        while fast != None:
-            fast, slow, prev = fast.next, slow.next, slow
-        if prev == None:
-            return head.next
-        prev.next = prev.next.next
-        return head
+        dummy = ListNode(-1)
+        dummy.next = head
+        left, right = dummy, dummy
+        while n:
+            right, n = right.next, n-1
+        while right.next: 
+            right = right.next
+            left = left.next
+        left.next = left.next.next
+        return dummy.next