September 2008
The minimum number is 11 as illustrated below. This problem is from Sam Loyd via Martin Gardner.
1111111AAAAAA
1111111AAAAAA
1111111AAAAAA
1111111AAAAAA
1111111AAAAAA
1111111AAAAAA
1111111244455
BBBBBB3344455
BBBBBB3344466
BBBBBB7777866
BBBBBB7777999
BBBBBB7777999
BBBBBB7777999
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