Get the indices of an array after sorting
Dyalog APL, 1 byte

⍋


Dyalog APL has a built in operator function (thank you Zacharý for clearing this up) to do this.

Example

⍋11 2 4 15
    2 3 1 4  
{⍵[⍋⍵]}11 4 2 15
    2 4 11 15


Here I'm indexing into the list by the sorted indices to return the list in ascending order.