Free public beta until December 4, 2026: every course is open, no card needed. After that, the first chapter of each course stays free and full access is a one-time $39 ($19 for up to 100 founding members chosen for helpful feedback). Your progress is kept. How the beta works

logarithmic time

Work that grows by about one step each time the data doubles.

TreeMap and TreeSet lookups and binary search; about 20 steps for a million elements.