Alice got a random number generator, The random number generated is between 1 to n equiprobably.
Now alice wants to know the expected number of turns to generate k distinct numbers.
Return the answer modulo 10^9 + 7.
I only have constraints for this. No sample input and output.
1<=k<=n <= 10^5

EDIT: My approach is to generate random numbers till I get k distinct elements and the answer will be the number of tries it had taken.
I doubt whether this is correct or not.

How to approach these types of questions?
Need algorithm for this. Preferrably a cpp answer.