Background

Around 1981, a British computer magazine (Personal Computer World) had a programming contest and this was one of the puzzles.

There are many 9-digit integers in the range 123456789 to 987654321 where each digit only ever appears once. What is the 100,000th number in this sequence?

Example

The first number is 123456789, the second is 123456798, the third is 123456879 and so on. No digit can repeat so 122345675 is not a valid number in this sequence.

The problem was “Write a program in C or C++ that outputs the 100,000th number as fast as possible. Use any algorithm, except you cannot pre-calculate the answer and then write a program that just prints the result (Somebody actually tried it). Your entry must calculate the number!”.  I’ll give this a month so the deadline is August 14th 2020. Please don’t copy any of the existing answers- you can with a bit of poking around and yesterday’s blog entry find some answers to this.

Prize?

Er fame and glory and a mention on here!

Note

It’s possible to solve this by hand. I did, back in 1981. It took me 20 minutes with a pen and paper. I also wrote a 6502 assembly language version of it than ran on a Vic-20. It took something like 10 hours to count up to the correct answer.

The reason I mention it is because I was looking at the backup of cplus.about.com, a website that I managed (curated, wrote tutorials etc.) on “Programming in C, C++, C#” (and even Go) between 2006 and 2013.  One of the things I ran there was a set of programming challenges and by the end I’d done a whopping 70 of them. I came up with the idea, published it, then adjudicated entries.

This page has a list of 65 of them.  There may be more from a later date but no more than 70.

As I can now gain access to most of them, I’m going to rerun a few here. starting with the first which I’ll publish tomorrow.