  We present two on-line algorithms for maintaining a topological order of a
directed $n$-vertex acyclic graph as arcs are added, and detecting a cycle when
one is created. Our first algorithm handles $m$ arc additions in $O(m^{3/2})$
time. For sparse graphs ($m/n = O(1)$), this bound improves the best previous
bound by a logarithmic factor, and is tight to within a constant factor among
algorithms satisfying a natural {\em locality} property. Our second algorithm
handles an arbitrary sequence of arc additions in $O(n^{5/2})$ time. For
sufficiently dense graphs, this bound improves the best previous bound by a
polynomial factor. Our bound may be far from tight: we show that the algorithm
can take $\Omega(n^2 2^{\sqrt{2\lg n}})$ time by relating its performance to a
generalization of the $k$-levels problem of combinatorial geometry. A
completely different algorithm running in $\Theta(n^2 \log n)$ time was given
recently by Bender, Fineman, and Gilbert. We extend both of our algorithms to
the maintenance of strong components, without affecting the asymptotic time
bounds.
