Problem - Hamiltonian Flights

Problem: There are $n$ cities with $m$ one-way flights. Count the number of routes from city $1$ to city $n$ that visit every city exactly once.

Link: CSES 1690

Constraints:

• $2 \leq n \leq 20$
• $1 \leq m \leq n^2$

Note: This is Hamiltonian path counting. The small $n$ suggests bitmask DP. The state is (current city, set of visited cities). Transition: move to an adjacent unvisited city and add it to the visited set.