for v in V do if deg[v] = 0 then q.add(v)
while not q.empty() do
  v = q.get()
  if color[v] = -1 then
    for uv in E do
      if color[u] = 0 then
        color[u] = 1
        q.add(u)
  else
    for uv in E do
      deg[u] -= 1
      if deg[u] = 0 then
        color[u] = -1
        q.add(u)