Topological sort not producing the expected output

Python Help
1

Course Schedule problem where each courses should be listed according to pre-requisites. The output list sequence should always start with no pre-requisite courses and any course in the sequence should have already met the prerequisites before it. Using topological sort on DAG (Directed acyclic graph) I am not getting the expected output.

courses_and_prereqs_in_list = {
“algorithms”: [“data structures”],
“calculus”: [“linear algebra”],
“compilers”: [“data structures”, “formal languages”, “computer organization”],
“data structures”: [“discrete math”],
“databases”: [“data structures”],
“discrete math”: [“intro to programming”],
“formal languages”: [“discrete math”],
“networks”: [“operating systems”],
“operating systems”: [“data structures”, “computer organization”],
“programming languages”: [“data structures”, “computer organization”],
}

##My Code

from collections import defaultdict

course = ['computer organization', 'intro to programming', 'calculus', 'discrete math', 'formal languages', 'programming languages', 'networks', 'algorithms', 'databases', 'linear algebra', 'data structures', 'compilers', 'operating systems']


class Graph:
    def __init__(self, vertices):
        self.graph = defaultdict(list)
        self.V = vertices
    
    def addEdge(self, u, v):
        self.graph[u].append(v)

    def neighbor_gen(self, v):
        for k in self.graph:
            yield k
    
    def topologicalsortutil(self, v, visited, stack):
        working_stack = [(v, self.neighbor_gen(v))]
        while working_stack:
            v, gen = working_stack.pop()
            visited[v] = True

            for next_neighbor in gen:
                if not visited[next_neighbor]:
                    working_stack.append((v,gen))
                    working_stack.append((next_neighbor, self.neighbor_gen(next_neighbor)))
                    break
            else:
                stack.append(v)

    def topologicalsort(self):
        visited = [False]*self.V
        stack = []
        for i in range(self.V):
            if not (visited[i]):
                self.topologicalsortutil(i, visited, stack)
        stack.reverse()
        return stack


g = Graph(13)
g.addEdge(11, 0)
g.addEdge(12, 0)
g.addEdge(5, 0)
g.addEdge(3, 1)
g.addEdge(2, 9)
g.addEdge(11, 10)
g.addEdge(11, 4)
g.addEdge(8, 10)
g.addEdge(7, 10)
g.addEdge(6, 12)
g.addEdge(4, 3)
g.addEdge(10, 3)

Need help in correcting the code to get

2

If this isn’t homework, why not just use one of these?