Ordering
Ordering[list] gives the positions that would sort the list. Ordering[list, n] gives positions of the n smallest elements.
Examples
Find sorting order:
list = {3, 1, 4, 1, 5, 9}
Ordering[list]
(* {2, 4, 1, 3, 5, 6} *)
(* Verify: list[[Ordering[list]]] is sorted *)
list[[Ordering[list]]]
(* {1, 1, 3, 4, 5, 9} *)
(* Positions of 3 largest *)
Ordering[list, -3]
(* {6, 5, 3} *)Please visit the official Wolfram Language Reference for more details.