Derivations

Interview problems worked from first ideas - written solution and video, free.

New problems are added regularly.

Handwritten counting bound for the adversarial hat problem: 25 times 2 to the 24, over 2 to the 25, equals 12.5

Coin flips give E[X] = 1 and a 25% chance that nobody walks. Parity guarantees 12 out of 25, and a counting argument proves you can't do better.