Add typing

Author: jeremytan-stripe

Diff for: sorts/topological_sort.py

@@ -9,7 +9,7 @@
 vertices = ["a", "b", "c", "d", "e"]
 
 
-def topological_sort(start, visited, sort):
+def topological_sort(start: str, visited: list, sort: list):
     """Perform topological sort on a directed acyclic graph."""
     current = start
     # add current to visited