Sudoku puzzle designers claim there are four levels of difficulty some
classify as gentle, moderate, tough, and diabolical. Some newspapers use
different terms.
The level of difficulty does not depend only on how many numbers are given
at the start.
Aside from the given numbers, three things determine the puzzle’s
difficulty: how many squares can be filled up by using the process of
elimination, how many guesses are needed to reach the right solution, and how
many times one has to backtrack to solve the puzzle.
A computer that generates these puzzles also classifies the puzzle’s
difficulty based on these four factors.
As I have experienced, the rating assigned by the computer is sometimes not
reliable. The possible reason is that the computer assumes all possible wrong
guesses before arriving at a final solution. However, after playing the puzzle
several times, I learned that luckily making the right guess the first time
allowed me to quickly solve the puzzle.
Some puzzles with a few given numbers are easy to solve, while others with
as many as twenty-seven given numbers are very difficult.