title: Python algorithm to solve triomines puzzle with backtracking
body: <p>Let's say I have a board like this</p>
  
  <pre><code>. . x . . .
  . . . . . .
  . . x . . x
  </code></pre>
  
  <p>x is used box and '.' are free. I need to put triomines to fill all the area, so there will be no free cells. Triomines are L-shaped, and I mark same triomino with the same number.</p>
  
  <p>So the solution could be like this:</p>
  
  <pre><code>1 1 x 3 5 5
  1 2 3 3 4 5
  2 2 x 4 4 x
  </code></pre>
  
  <p>What the possible backtracking python algorithm might be?</p>