|We do not do your homework: it is set for a reason. It is there so that you think about what you have been told, and try to understand it. It is also there so that your tutor can identify areas where you are weak, and focus more attention on remedial action.
Try it yourself, you may find it is not as difficult as you think!
But without giving you any code, I'll tell you how I'd do it: I'd create a struct to hold each distinct value and a sorted list of indexes in the array. I'd then fill out a collection of such structs - one for each distinct value in the array - and traverse the collection when complete to find the largest difference in indexes.
If you meet a specific problem, then please ask about that and we will do our best to help. But we aren't going to do it all for you!
Bad command or file name. Bad, bad command! Sit! Stay! Staaaay...