Invariants

Puzzles

• djikstras.urnAn urn contains an unknown number of white and black balls (but more than one).
• infected.checkerboardShow that fewer than N black cells can never turn a whole NxN grid black when each cell with at least 2... New
• odd.handshakesn people (n >= 2) attend a party, and each person shakes hands with some of the others. New