Brutal

 Just solved a 'Brutal' Sudoku puzzle, which is the maximum difficulty for the

Math is Fun site. Used the hints function to identify the sure things at the beginning,

thus avoided tiring myself out. This should not be an endurance test. From there,

logically found the rest. Many spots had multiple possible answers, but one could

find the only one in the lot that satisfied all the conditions, coming down to 'this

is the only spot I could eventually put a '1' or whatever!!

                                                          

Clearly, all of these Sudos could be presented from an already complete board

on which one clears a certain number of tiles. If the player adds a value, computer

knows if it is correct and can output 'solvable' or 'not solvable'. Generating the hints

comes down to the most elementary check with respect to horizontal and vertical 

values...