http://en.lichess.org/training/33484
The puzzle says the mating sequence starting with Qd2+ is the best mating sequence, but I think the sequence starting with Ng4+ is shorter:
Ng4+ Kg1 Qc5+ Kf1 Qf2#
Ng4+ Kf1 Qd1#
Ng4+ Ke1 Re8+ Ne5 Rxe5+ Kf1 Qd1#
Ng4+ Ke2 Re8+ Ne5 Rxe5+ Kf1 Qd1#
The puzzle says the mating sequence starting with Qd2+ is the best mating sequence, but I think the sequence starting with Ng4+ is shorter:
Ng4+ Kg1 Qc5+ Kf1 Qf2#
Ng4+ Kf1 Qd1#
Ng4+ Ke1 Re8+ Ne5 Rxe5+ Kf1 Qd1#
Ng4+ Ke2 Re8+ Ne5 Rxe5+ Kf1 Qd1#