Backtracking sudoku solver.
A sudoku board is described as a list of lists. Zeros represent empty squares.
board=[
[9, 0, 0, 7, 0, 0, 0, 0, 0],
[0, 0, 0, 5, 1, 0, 3, 0, 0],
[0, 5, 4, 0, 0, 0, 0, 9, 0],
[0, 7, 0, 0, 3, 1, 0, 8, 5],
[1, 0, 0, 0, 9, 0, 0, 0, 0],
[0, 0, 0, 0, 6, 0, 0, 0, 0],
[0, 0, 0, 0, 0, 8, 9, 0, 6],
[7, 2, 0, 0, 0, 0, 0, 0, 0],
[0, 0, 0, 0, 0, 2, 0, 0, 3]
]