There are following answers given as correct ones:
1) 5, 6, 3, 4, 0, 1, 2
2) 6, 5, 3, 4, 0, 1, 2
3) 5, 6, 4, 3, 0, 2, 1
4) 6, 5, 4, 3, 0, 1, 2
5) 5, 6, 3, 4, 0, 2, 1
6) 5, 6, 3, 4, 1, 2, 0
but it appears from the graph picture that the 5, 6, 4, 3, 2, 1, 0 should also be a correct order, no?
Course: Grokking the Coding Interview: Patterns for Coding Questions - Learn Interactively
Lesson: Topological Sort (medium) - Grokking the Coding Interview: Patterns for Coding Questions