title: checking if a directed graph has only single one topological sort
body: <p>I'm trying to write pseudo-code for an algorithm that suppose to check whether a directed graph has only single one topological ordering. I've already come up with the pseudo-code for a topological sort (using DFS), but it does not seem to help me much. I wonder if there is no sinks in that graph -then  there's not a single one topological ordering (might it help?).</p>