# Clean and commented Python solution

• ``````class Solution(object):
"""
This is similar to finding a cycle in a graph and at the same time,
doing a Topological sort of the vertices. The approach here uses DFS.
"""
def findOrder(self, numCourses, prerequisites):
"""
:type numCourses: int
:type prerequisites: List[List[int]]
:rtype: List[int]
"""
#If there are no dependencies among courses, they can
#be taken in any order. Weird input/output case tbh.
if prerequisites == []:
return [i for i in xrange(numCourses)]

#Convert the edges into an adjecency list representation
self.adj = [set() for i in xrange(numCourses)]

for i,j in prerequisites:

#visited set will track vertices seen while exploring the current vertex
self.visited = set()

#completed set will track vertices that have been completely explored
self.completed = set()

self.res = []

#For every vertex that has not been explored, visit it
if i not in self.visited and i not in self.completed:
possible = self.visit(i)

#visit() returns False if a cycle is detected
if not possible:
return []

#Topological sort is the reverse of the order in which the
#vertices were explored
return self.res[::-1]

def visit(self, u):
#mark the current vertex as visited
possible = True

#For every vertex adjecent to v

#explore the vertex only if not already explored
if v not in self.visited and v not in self.completed:
possible = possible and self.visit(v)

#if this vertex was seen during the exploration of current vertex,
#then there is a cycle in the graph and we can return False
if v in self.visited:
possible = False
break

#finally, we can mark the current vertex as completely explored
self.visited.remove(u)