Place one of the four letters A, B, C or D in each of the blue-coloured cells. No letter can be horizontally or vertically adjacent to itself. The yellow-coloured tables above and to the left of the grid indicate how many times each letter appears in that column or row.

This is an extended version of ABCD.

				A	3	2	0	2	2	2	0	3	1
				B	2	1	3	0	0	1	3	0	2
				C	1	2	1	3	2	3	1	2	1
	A	B	C	D	0	1	2	1	2	0	2	1	2
	2	0	4	3
	4	4	1	0
	3	2	2	2
	3	3	2	1
	1	0	4	4
	2	3	3	1

I see Charlie found the same solution.  I had considered writing a program, but found it simple enough to do by trial and error. The key seems to be to start with the letters which occur most often in a given line (row or column), since there are limited possible placements.  It went a bit slowly at first, but when about half determined, the rest fell into place.

I normally prefer a computer solution, since (unless a bug) it will show a given (trial and error) solution is in fact unique; I'll look over Charlie's program.

Posted by ed bottemiller on 2008-03-07 13:04:34