a session with tsort command involving topological sorting

$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