It causes me unspeakable agony to see that <a href="/2013/04/17/why-sudoku-is-boring/#more-1744" target="_blank">my post about why sudoku is boring</a> is one of the most frequented posts in this blog, mostly because most of my readers clearly disagree with the title. I recently received an email titled <i>"why sudoku is not all that boring"</i> by an old friend, and he taunted me that the sudoku ```python S = [ 0, 0, 0, 0, 6, 0, 0, 8, 0, 0, 2, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 7, 0, 0, 0, 0, 1, 0, 2, 5, 0, 0, 0, 3, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 4, 0, 0, 0, 0, 4, 2, 0, 1, 0, 0, 0, 3, 0, 0, 7, 0, 0, 6, 0, 0, 0, 0, 0, 0, 0, 0, 0, 5, 0 ] ``` would take my 5-minute hack of a backtracking algorithm ```bash real 51m3.656s user 50m32.260s sys 0m2.084s ``` to solve. So, it seems like some sudokus are really hard, even for a computer, right? <b>Wrong wrong wrong wrong wrong</b>. <a href="https://blag.nullteilerfrei.de/2014/07/03/why-someone-thought-that-sudoku-might-not-be-boring-while-actually-you-should-learn-how-to-properly-implement-backtracking/#more-2945" class="more-link">Read how to implement backtracking properly.</a>