Given an adjacency matrix or other data based on the nodes, there are many algorithms for determining whether the graph is acyclic (e.g. row reduction or leaf trimming).

If I have only a set of edges (e.g. ${(a,b), (b,c), (d,b)}$), I could obviously convert that into an adjacency matrix and proceed from there.

I'm wondering if there is a good algorithm that works directly with the edges.


1 Answer 1


You could use a disjoint set (union-find) data structure and iterate through the edges one by one. For each edge $(u, v)$, if the two vertices already belong to the same subset, then the graph is not acyclic. Otherwise, you merge the subset containing $u$ with the subset containing $v$. If all edges are added successfully, the graph is acyclic.

This runs in $O(E\alpha(V))$, where $V$ is the number of vertices, $E$ is the number of edges and $\alpha$ is the inverse Ackermann function.

In principle, however, I don't think that this is any faster than iterating through the edges to build an adjacency list and then use that for determining if the graph is acyclic.

  • $\begingroup$ "I don't think that this is any faster than …". Right. Some people have trouble relating algebraic manipulation to "reality". I wanted a graphical rather than an algebraic solution. And this is it. Thanks. $\endgroup$ Mar 10 at 14:27

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