Hello Codeforces!

I am facing this problem and don't know how to avoid TLE. Can someone please give me an algorithm hint for this problem? I already know the O(n^2) way.

Here is the problem:

We have n numbers which are the lengths of the sticks. Find the number of unique triangles that can be formed from the given n sticks. Unique triangles are understood as triangles that do not have a pair of 3 identical sides. Note that n<=5000 and each stick's length is <= 1e9

Thank you.

Note: I can solve it know, thanks.