Envío 3645

Código

from collections import Counter, deque


def get_in_degree_vertices(graph):
    in_degree_vertices = Counter({key: 0 for key in graph.keys()})

    for vertex in graph:
        for neighbour in graph[vertex]:
            in_degree_vertices.update([neighbour])

    return in_degree_vertices


def graph_has_cycle(graph, vertex_num):
    in_degree_vertices = get_in_degree_vertices(graph)
    queue = deque(filter(lambda v: v == 0, in_degree_vertices.values()))

    number_of_visited_vertices = 0
    while len(queue):
        vertex = queue.popleft()
        number_of_visited_vertices += 1
        neighbours = graph[vertex] if vertex in graph else []

        for neighbour in neighbours:
            in_degree_vertices.subtract([neighbour])

            if in_degree_vertices[neighbour] == 0:
                queue.append(neighbour)

    return number_of_visited_vertices < vertex_num


vertex_num, edge_num = [int(x) for x in input().split()]
adjacency_list = {}

for _ in range(edge_num):
    from_vertex, to_vertex = [int(x) for x in input().split()]
    if from_vertex in adjacency_list:
        adjacency_list[from_vertex].append(to_vertex)
    else:
        adjacency_list[from_vertex] = [to_vertex]

result = False if edge_num == 0 else graph_has_cycle(
    adjacency_list, vertex_num)
print("Yes" if result else "No")