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Sunday common C++ interview problem: Shortest path to all keys.

Given a map represented as std::vector<std::vector<char>>, where:

- # are walls
- @ is the entrance
- the first k lowercase letters are keys
- corresponding uppercase letters are locked doors

Assume k <= 6 and return the shortest path length that collects all keys (-1 if none).

Solve it yourself: https://godbolt.org/z/9dW6Gzrer
Solution: https://godbolt.org/z/rqbEe3xKv