Past exam of the year 1 of the computer science course of the University of Oxford / 2016 / Functional programming, design and analysis of algorithms / 6 / c / Solution

A topological sort is a linear ordering of all vertices of a Directed acyclic graph in which every directed edge points from an earlier vertex to a later one. For example, with $V=\{a,b,c\}$ and $E=\{(a,c),(b,c)\}$, both $(a,b,c)$ and $(b,a,c)$ are topological sorts. They differ because neither edge nor directed path constrains the relative order of $a$ and $b$.