This is a really nice puzzle! I didn't know William Wu wrote a paper about it. I saw it many years ago on William Wu's riddles site[1] (highly recommended). It'

This is a really nice puzzle! I didn't know William Wu wrote a paper about it. I saw it many years ago on William Wu's riddles site[1] (highly recommended). It's one of the most popular problems there. The original thread[2] in the forum has 25 pages, covering lots of approaches. There is also another thread[3] summarizing the discussion. [1] http://wuriddles.com/ http://wuriddles.com/ [2] http://www.ocf.berkeley.edu/~wwu/cgi-bin/yabb/YaBB.cgi?board=riddles_hard;action=display;num=1027805293 http://www.ocf.berkeley.edu/~wwu/cgi-bin/yabb/YaBB.cgi?board... [3] http://www.ocf.berkeley.edu/~wwu/cgi-bin/yabb/YaBB.cgi?board=riddles_hard;action=display;num=1236693480 http://www.ocf.berkeley.edu/~wwu/cgi-bin/yabb/YaBB.cgi?board...