$tsort 1 tsort: -: input contains an odd number of tokens $tsort 1 2 1 2 $tsort 0 1 2 3 4 5 0 2 4 1 3 5 $tsort 0 1 2 3 4 5 0 2 4 1 3 5 $ http://www.geeksforgeeks.org/topological-sorting/ https://www.quora.com/What-are-some-real-world-applications-of-topological-sort