Write a program to solve a Sudoku puzzle by filling the empty cells.
A sudoku solution must satisfy all of the following rules:
<li>Each of the digits <code>1-9</code> must occur exactly once in each row.</li>
<li>Each of the digits <code>1-9</code> must occur exactly once in each column.</li>
<li>Each of the the digits <code>1-9</code> must occur exactly once in each of the 9 <code>3x3</code> sub-boxes of the grid.</li>
Empty cells are indicated by the character '.'.
A sudoku puzzle...
...and its solution numbers marked in red.
Note:
<li>The given board contain only digits <code>1-9</code> and the character <code>'.'</code>.</li>
<li>You may assume that the given Sudoku puzzle will have a single unique solution.</li>
<li>The given board size is always <code>9x9</code>.</li>