Mine was: step 1: which number is never shown after others? it must be first. now which number is never shown except after that one? etc. so it took about 30 se

Mine was: step 1: which number is never shown after others? it must be first. now which number is never shown except after that one? etc. so it took about 30 seconds in my head. The trouble is I spent fifteen minutes before this in an absolute rage at how OP, and then with commenters here backing him up, seemed to be answering something different to what the question asked because I can't read "determine the shortest possible secret passcode of unknown length" anything other than a weird way of asking "what is the minimum length". My mind just locked in to my wrong understanding. I think my problem solving was better than the people spinning up elaborate code, but it seems at the cost of lateral thinking needed for "problem understanding". please note that several of the commenters today are accounts created just to praise OP