I wrote a sudoku solver, that will create 9x9 array of arrays filled with numbers 1-9 for "blank" fields and only the number in defined field. Then for each square with only one number, I removed the number in that square from all arrays in row, column and subsquare. That typically left me with a solved array, but will contain all possible results for "guessing" fields. It was written in php (the thing I leaned at the time) and ran too fast for me to bother measuring, less than a second. Since I've "solved all sudokus" they stopped being fun.