A simple sudoku solver that uses a recursive backtracking algorithm. I store sudoku boards as 9x9 arrays of 8-bit integers and sets of used numbers in each row, column, and square as arrays of 16-bit integers.

On my machine it solves this board in approximately 60ms. The example from Wikipedia takes 240 nano seconds to complete.

I experimented with not copying the board on every iteration and with parallelizing checking each legal move, but they turned out to be slower.