algorithm - Proof of detecting the start of cycle in linked list

Question

(This question already has answers here): question from:https://stackoverflow.com/questions/3952805/proof-of-detecting-the-start-of-cycle-in-linked-list

Answer 1

You can make your 'Find the start of cycle' proof simpler if you don't use meeting point.
Let second pointer start not at the 'meeting point', but M steps ahead of first pointer. (Where M is the length of the loop.)

This way, proof is pretty obvious. When the first pointer reaches start of the loop, second will be exactly M steps ahead: also at the start of the loop.