A topological sort is a permutation 
of the vertices of a graph such
that an edge 
implies that 
appears before 
in 
(Skiena 1990, p. 208). Only acyclic digraphs
can be topologically sorted. The topological sort of a graph can be computed using
TopologicalSort[g]
in the Wolfram Language package Combinatorica`
.
Topological Sort
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References
Skiena, S. "Topological Sorting." §5.4.3 in Implementing Discrete Mathematics: Combinatorics and Graph Theory with Mathematica. Reading, MA: Addison-Wesley, pp. 208-209, 1990.Referenced on Wolfram|Alpha
Topological SortCite this as:
Weisstein, Eric W. "Topological Sort." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/TopologicalSort.html