Pattern Recognition Drill
Medium Linked Lists
The Problem
Find the node where the cycle begins in a linked list.
What approach would you use?
Think about it before scrolling down.
After detection: reset slow to head, advance both by 1. They meet at cycle start. O(n).
Store each visited node; first repeat is cycle start. O(n) space.
Common Trap
This is a two-phase algorithm: detect first, then find start. Don't try to do both in one pass.