Finding loop in a singly linked-list

Question

How can I detect that whether a singly linked-list has loop or not?? If it has loop then how to find the point of origination of the loop i.e. the node from which the loop

Answer 1

The selected answer gives an O(n*n) solution to find the start node of the cycle. Here's an O(n) solution:

Once we find the slow A and fast B meet in the cycle, make one of them still and the other continue to go one step each time, to decide the perimeter of the cycle, say, P.

Then we put a node at the head and let it go P steps, and put another node at the head. We advance these two nodes both one step each time, when they first meet, it's the start point of the cycle.