Floyds-Cycle-Finding-Algorithm

LeetCode 142 note: after fast and slow meet, send one back to head and step both one at a time; they meet again at the cycle entry

LeetCode 202 note: repeatedly take the sum of squared digits, which reaches 1 or enters a cycle; detect it with a set of seen numbers or fast and slow pointers

LeetCode 141 note: Floyd’s cycle detection, fast two steps and slow one; with a cycle they must meet, each advancing from its own position