That’s a different (rather easier) problem: note that the safe combination can contain the same digit more than once. The solution to this problem is given by

That’s a different (rather easier) problem: note that the safe combination can contain the same digit more than once. The solution to this problem is given by de Bruijn sequences: https://en.wikipedia.org/wiki/De_Bruijn_sequence https://en.wikipedia.org/wiki/De_Bruijn_sequence