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Help in understanding example — Graph

Revision en2, by niranjan_kumar, 2021-06-12 15:55:46

Today, there was a test conducted by Uber and it's time slot was 8 AM to 2 PM (IST), so it has finished now. I need help in understanding the answer to a sample case of one of the problem. Problem was something like this:

There are N cities with a road between every pair of cities. Calculate the number of ways (modulo 1e9+7) such that you start travelling from city 1 and return to the same city 1 after completing exactly K trips where one trip means travelling from a current city to any other city.

Constraints: N <= 1e9; K <= 1e6

My approach: We start with city 1, now we have option to go (N-1) cities for 1st trip, then again (N-1) for 2nd trip, and so on. Hence, in short we have options of (N-1) cities for first (K-2) trips, then for the 2nd last trip we can go to (N-2) cities and for the last trip we have only 1 option that is to return to city 1. Hence according to this, answer should be: (N-1)^(K-2) * (N-2)

Sample Case: N=4, K=4

With my approach I am getting answer as 18. But according to author the answer should be 21. I need your help in understanding how there are 21 ways. I am not getting answer as 21 even when I try to find the ways using a pen and paper.