Uncleared programming question.

See more:

C++

C

C#

algorithm

Problem Description
Alice has got a Random Number Generator. The generator generates a random number from 1 to N equiprobably.
Now Alice wants to know the expected number of turns until K distinct elements are generated.
Help Alice find this value modulo 10⁹+7.

Problem Constraints
1 ≤ K ≤ N ≤ 10⁵

Input Format

Input consists of 2 arguments, N=A and K=B in this order.

Output Format
Return a single integer, the expected value modulo 10⁹+7

Example Input
  Input 1:

    N = 2
    K = 1

  Input 2:

    N = 2
    K = 2

Example Output
  Output 1:

    1

  Output 2:

    3

Example Explanation
  Explanation 1:

Whatever number is generated in the first turn will be unique and thus the answer is 1

  Explanation 2:

There are 2 cases:
First number generated is 1. The expected turns for getting a 2 later is 2.
First number generated is 2. The expected turns for getting a 1 later is 2.
So, final expected value is 1 + (2+2)/2 = 3.

What I have tried:

I tried to understand the question from 15th sep 2020 till now but, I am unable to understand the question.
I was stuck because the sample inputs looks easy but consider inputs as N=3 and K=2 the output for this is 500000006
Please explain if any one understood the question. I am not expecting the complete answer but a way to solve the problem.
Thank you.