story_teller_topological_sorting_dfs

from collections import deque


def top_sort_dfs(node, sorted_nodes, graph, visited, cycles):
    if node in cycles:
        raise Exception
    if node in visited:
        return
    visited.add(node)
    cycles.add(node)

    for child in graph[node]:
        top_sort_dfs(child, sorted_nodes, graph, visited, cycles)
    sorted_nodes.appendleft(node)
    cycles.remove(node)


graph = {}
visited = set()
cycles = set()
sorted_nodes = deque()

while True:
    line = input().split('->')
    if 'End' == line[0]:
        break
    if len(line) == 1 or line[1].strip() == '':
        graph[line[0].strip()] = []
    else:
        graph[line[0].strip()] = line[1].strip().split(' ')
try:
    for node in graph:
        top_sort_dfs(node, sorted_nodes, graph, visited, cycles)
    print(*sorted_nodes)
except Exception:
    print('Invalid topological sorting')