algorithm - Find cycle of shortest length in a directed graph with positive weights

Question

I was asked this question in an interview, but I couldn't come up with any decent solution. So, I told them the naive approach of finding all the cycles then picking the cycle with the least length.

I'm curious to know what is an efficient solution to this problem.

Answer 1

You can easily modify Floyd-Warshall algorithm. (If you're not familiar with graph theory at all, I suggest checking it out, e.g. getting a copy of Introduction to Algorithms).

Traditionally, you start path[i][i] = 0 for each i. But you can instead start from path[i][i] = INFINITY. It won't affect algorithm itself, as those zeroes weren't used in computation anyway (since path path[i][j] will never change for k == i or k == j).

In the end, path[i][i] is the length the shortest cycle going through i. Consequently, you need to find min(path[i][i]) for all i. And if you want cycle itself (not only its length), you can do it just like it's usually done with normal paths: by memorizing k during execution of algorithm.

In addition, you can also use Dijkstra's algorithm to find a shortest cycle going through any given node. If you run this modified Dijkstra for each node, you'll get the same result as with Floyd-Warshall. And since each Dijkstra is O(n^2), you'll get the same O(n^3) overall complexity.