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I was recently talking to one of my friends who played chess (much better than me, I'll admit). He said he had a different kind of chess problem, one that may be a little easier, or that may be a little bit harder than usual. He said it was a chess lock puzzle.

He then grabbed two(!) chess sets and set up the following position:

enter image description here

He challenged me to find the least number of moves to capture the black king, using the following rules:

  1. White gets an infinite number of moves.
  2. White may only capture undefended pieces, and the black king (checkmate isn't a thing, just capture the black king)
  3. Rooks are the only keys that work. You must capture the black king with a rook.

He said his best was to do it in 17 moves. Can you do it quicker?

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    $\begingroup$ Just to clarify, do black's pieces capture whites pieces if they are in a position where they could? $\endgroup$
    – gabbo1092
    Commented Oct 30, 2018 at 15:10
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    $\begingroup$ @gabbo1092 no, but white is not permitted to capture black pieces that are guarded. $\endgroup$
    – Excited Raichu
    Commented Oct 30, 2018 at 15:11
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    $\begingroup$ @gabbo1092 I think that black pieces can't do any moves. We only consider white moves. Am I right? $\endgroup$
    – Piotr Wasilewicz
    Commented Oct 30, 2018 at 15:11
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    $\begingroup$ Might Black have moved in the past, even though Black will never move again? If so, depending upon what Black just did, there could be a four-move solution (1. dxe e.p. or 1. fxe e.p., followed by 2. e7 3.e8=R 4. Rxa8) $\endgroup$
    – supercat
    Commented Oct 30, 2018 at 23:00
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    $\begingroup$ @supercat remember, my friend set up the position, with no prior moves. I knew someone would try that ;) $\endgroup$
    – Excited Raichu
    Commented Oct 30, 2018 at 23:32

4 Answers 4

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I think there's a quicker way to do it.

Rb2 Nb1 Rd2 Bd1 Rf2 B3e2 Rf4 Bgf3 Rg4 Rxg7 Rg8 Rxa8

for 12 moves.

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  • $\begingroup$ Yeah, this was it (and this gets the check for the earlier timestamp) $\endgroup$
    – Excited Raichu
    Commented Oct 30, 2018 at 15:27
  • $\begingroup$ Darn I was just finishing double checking mine and you just beat me! Congrats! $\endgroup$
    – gabbo1092
    Commented Oct 30, 2018 at 15:27
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    $\begingroup$ Move 6 is ambiguous as written. You should replace the 'f' by a '3'. $\endgroup$
    – Evargalo
    Commented Oct 30, 2018 at 23:38
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I think I found a solution in 12 moves:

1.Rb2
2. Nb1
3. Rd2
4.Bd1
5. Rf2
6.Be2
7.Rf4
8.Bf3
9.Rg4
10. Rxg7
11.Rg8
12.Rxa8

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    $\begingroup$ I think you were about 28 seconds behind @Especially Lime unfortunately $\endgroup$
    – JGibbers
    Commented Oct 30, 2018 at 15:26
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I can do it in 15 moves by my count.

The key is to move it around like one of those sliding block puzzles (like rush hour first. Then you only need to get a pawn promoted to a rook. No need to move any existing rooks out.

Spoiler Break

Rb2
Nb1
Rd2
Bd1
Ne2
Nf4
Ne6
Nxg7
Ne8
Nxf6
Ng8
f6
f7
f8=R
Rxa8

And will look something like this (apologies if I mess something up, I set the picture up after the fact, the annotation should be right.

https://i.sstatic.net/PrcVl.png

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I think it can be done in 10 moves:

1.Rb2 2.Nb1 3.Rd2 4.Bd1 5.Be2 6.Nf3 7.Ng5 8.Ne6 9.Nc7 10.Na8

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    $\begingroup$ Make sure to look at Rule 3, this would be perfectly valid if that wasn't there. $\endgroup$
    – Excited Raichu
    Commented Oct 30, 2018 at 16:34

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