11446 - Where is the 'back' button?

[sclo]

Oh, no wonder I couldn't find any faster algorithm. I thought of a bruteforce O(n*2^n) algorithm long time ago, but thought that the time limit is too tight for it. Now I see that I can use bitmask to make it O(2^n), which should be fast enough.

[baodog]

Hi,

Nevermind...

[Khaled_912]

We choose some leaves and make sure every node could reach at least one of them.
(and choose some roots to make sure every node could be reached from them)

I don't quite understand what you mean by this, can you elaborate more please?