Detect a Loop in a Linked List

Question

## Problem

Determine whether a singly linked list contains a cycle, in O(1) extra space, and return the cycle's start.

## Example
```
Floyd's tortoise and hare
```

## Constraints
- 0 ≤ nodes ≤ 10^4

## What Fastly looks for
Reported Fastly question. Explain the meeting-point math for the cycle start.