ALGO(G,A,B)
Z=empty
INIT(G) -> in init metto due colori c1 e c2
for each b in B do
	c1[b]=n
for each a in A do
	dfs_visit(G,a,c1)
for each a in A do
	c2[a]=nero
for each b in B do
	dfs_visit(GT,b,c2)
for each v in V do
	if c1[v]=n && c2[v]=n then
		Z = Z u {v}
return Z