Hamiltonian Flights - Problem Statement

You will solve CSES 1690 - Hamiltonian Flights. This is NOT the Traveling Salesman Problem (TSP).

The difference: TSP finds the minimum cost Hamiltonian cycle (visit all cities, return to start). CSES 1690 counts Hamiltonian paths from city $1$ to city $n$ (visit all cities exactly once, fixed endpoints).

You'll use bitmask DP where $dp[mask][i]$ = number of ways to visit the cities in 'mask' and end at city $i$. The final answer is $dp[(1<<n)-1][n-1]$ modulo $10^9+7$.