How to determine whether a linked list contains a loop? [duplicate]

Answer 1

I had this exact problem in real live code last week.

All I did was kept a pointer (link) for the first element. Then as I iterated through the list, if I ever got that pointer again, I know there's a loop.

Answer 2

Probably the same technique as checking if a graph is a tree (trees don't get to have cycles), see this this question. It recommends either a topological sort or a depth first search.

Answer 3

Easy. Maintain two pointers into the list. At each step, advance one pointer by a single link, and advance the other by two links. Test to see if they point to the same element. If so, you have a loop. If not, repeat until you find a loop or you reach the end of the list.