ALGO(G,I,FG)
INIT(G)
GT=TRASPOSTA(G)
for each v in I do
	if(c1[v]=bianco)then
		dfs_visit(G,v,c1)
for each u in FG do
	if(c1[u]=nero)then
		dfs_visit(GT,u,c2)
for each v in I do
	if(c1[v]=nero || c2[v]=nero)then
		return true
	return false