The 25 cells below are connected so that are formed 5 strings of 5 cells. In each string you can read the letters A, B and C in this order (disregarding the empty cells in the string), and more, in the whole grid each letter appears in every row and in every column once, and only once.

             +---+ +---+ +---+ +---+ +---+
             | A |-|   | | B |-| C |-|   |
             +---+ +---+ +---+ +---+ +---+
               |           |
             +---+ +---+ +---+ +---+ +---+
             | B | | A |-|   | |   | | C | 
             +---+ +---+ +---+ +---+ +---+
               |                 |     |
             +---+ +---+ +---+ +---+ +---+
             |   |-| C | |   | | A | | B |
             +---+ +---+ +---+ +---+ +---+
                           |     |     | 
             +---+ +---+ +---+ +---+ +---+
             |   |-|   | | C |-| B | | A | 
             +---+ +---+ +---+ +---+ +---+
               |     |                 |
             +---+ +---+ +---+ +---+ +---+
             | C | | B |-| A | |   |-|   | 
             +---+ +---+ +---+ +---+ +---+

Fill the 7 x 7 grid below the same way (there are 7 strings, each one with 7 cells), using the letters A, B, C and D. One letter (D) is already placed.

 
       +---+ +---+ +---+ +---+ +---+ +---+ +---+
       |   |-|   |-|   |-|   |-|   | |   |-| D | 
       +---+ +---+ +---+ +---+ +---+ +---+ +---+
         |                       |     |     |  
       +---+ +---+ +---+ +---+ +---+ +---+ +---+
       |   | |   | |   |-|   | |   | |   | |   |
       +---+ +---+ +---+ +---+ +---+ +---+ +---+
               |     |     |           |     |
       +---+ +---+ +---+ +---+ +---+ +---+ +---+
       |   |-|   | |   | |   |-|   | |   | |   | 
       +---+ +---+ +---+ +---+ +---+ +---+ +---+
         |           |                 |
       +---+ +---+ +---+ +---+ +---+ +---+ +---+
       |   | |   | |   | |   |-|   | |   | |   | 
       +---+ +---+ +---+ +---+ +---+ +---+ +---+
         |     |     |           |           |
       +---+ +---+ +---+ +---+ +---+ +---+ +---+
       |   |-|   | |   | |   | |   | |   |-|   |
       +---+ +---+ +---+ +---+ +---+ +---+ +---+
                           |     |     |
       +---+ +---+ +---+ +---+ +---+ +---+ +---+
       |   | |   |-|   |-|   | |   | |   |-|   | 
       +---+ +---+ +---+ +---+ +---+ +---+ +---+
         |     |                 |           | 
       +---+ +---+ +---+ +---+ +---+ +---+ +---+
       |   |-|   | |   |-|   |-|   | |   |-|   | 
       +---+ +---+ +---+ +---+ +---+ +---+ +---+

- - C B A - D
D A B - - C -
- - A C D B - 
B - - D C A -
C D - - B - A 
A C D - - - B
- B - A - D C 

:)

Edited on November 18, 2005, 4:56 pm

Posted by Morgan on 2005-11-18 16:56:04