Graphs saved the Ten Queens

I pressed Enter and waited nervously for a second.

The number 724 was printed in the REPL below my function call:

>>> queens(10)
724

After a split second Queen #3 shouted “BRILLIANT!”. Her enthusiasm was welcome! But there really was no need to shout so close to my ears…

Queen #7 wasn’t so easily impressed. “How can we know this is correct?”.

I proceeded to explain:

“The problem isn’t that difficult. It may sound difficult… But it is not. I’ve solved many different problems like this one in my bootcamp. See–”

Queen #7 silenced me with her hand.

“If you have solved different problems how can they be like this one?”

I smiled. It was an excellent question. But a simple question. A question coming from someone who hasn’t solved many problems before. A question from someone who doesn’t do programming.

“To solve a problem you need to look beyond the words used to describe the problem.”

“Explain. Please.” Queen #7 was starting to show her impatience.

“You explained what you needed when we talked before. The ten of you wanted to be distributed on a board without clashing with each other.

After our conversation I had to think for a bit... And then the EUREKA moment struck.

Experience told me this was a problem I could solve with a graph searching algorithm. Experience that I obtained from solving problems like the ones from my bootcamp.

I have solved so many graph searching problems that I could have written the code with my eyes closed.

I didn’t do it because I wanted to avoid typos. But I could have.”

Queen #7 raised her eyebrow. My words were rubbing her the wrong way. I decided to correct course:

“I don’t mean that in an arrogant way. Let me put it another way.

You’ve been queen for many years. I am certain there are recurring problems your people need help with. – Queen #7 nodded – Aren’t recurring problems easier to solve?”

Queen #7 nodded again: “Whenever a bishop and a knight waltz in here I know what’s coming. It’s always the bishop that envies the knight or vice-versa.

Sometimes it’s the knight that wanted to move diagonally instead of in its weird way.
Sometimes it’s the bishop that wanted to ride a horse instead of having to walk.
But it’s always about envy.

It’s always the same problem.”

“Precisely.

You’ve been queen longer than I’ve been writing code. But I’ve practiced a lot. And this practice gave me the tools to solve this with 20 lines of code.”

Queen #7 smiled. I think. It’s hard to tell.

She then concluded: “Very well. Now I understand why you say this was like other different problems you’ve solved.

Walk me through your 20 lines of code.”

And that’s what I did.
I started explaining my 20 lines of code.

A couple of minutes later Queen #7 was satisfied.

She waved her hand and four pawns walked in.
They carried a beautiful chest with intricate engravings filled with gold coins.

I was puzzled and turned to Queen #7.

Queen #7 was definitely smiling.
“Payment for your help.”

If you want, you can see the code I showed the queen.

Here’s a link to the 20 lines of code that solves the “ten queens problem”.

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