Cycle Detection — Floyd's Tortoise & Hare

Algorithm — 3 phases

Phase 1: Move tortoise +1, hare +2. When they meet, a cycle exists.

Phase 2: Reset tortoise to head, keep hare at meeting point. Move both +1 until they meet — that's the cycle start μ.

Phase 3: Keep tortoise at cycle start, move hare +1 until it laps tortoise — that count is λ.