from collections import deque

# This is a solution which uses DFS to trace the paths; there
# are other solutions as well

classes = {
    'CPSC 230': {'CPSC 231'},
    'CPSC 231': {'CPSC 350', 'CPSC 330'},
    'CPSC 350': {'CPSC 380', 'CPSC 408', 'CPSC 406'},
    'CPSC 330': {'CPSC 351'},
    'CPSC 351': set(),
    'ENGR 101': set(),
    'CPSC 380': set(),
    'CPSC 406': set(),
    'CPSC 408': set()
}


def topological_sort(adjacency_list: dict[str, set[str]]):
    result = deque()
    visited = set()

    def dfs(current: str, visited_in_traversal=None):
        if visited_in_traversal is None:
            visited_in_traversal = set()
        if current in visited:
            return
        if current in visited_in_traversal:
            raise ValueError("Graph has a cycle")

        visited_in_traversal.add(current)

        for neighbor in adjacency_list[current]:
            dfs(neighbor, visited_in_traversal)

        # to trace the reverse path, use .append
        result.appendleft(current)
        visited.add(current)

    for node in adjacency_list.keys():
        if node not in visited:
            dfs(node)

    return result


print(topological_sort(classes))