Sudoku!

This is such a classic problem in code.  And I got in the puzzle-solving mood this afternoon after determining that the old "how many ways can a robot climb n stairs if he can take 1 or 2 steps at a time" is actually fibonacci(n) !

Anyway, so here's my sudoku, in ruby, of course. ;o)

def valid(num, x, y, solution)
  return false if (solution[y].include?(num))
  return false if (solution.map{|row|row[x]}.include?(num))
  (x-x%3).upto(x-x%3+2) { |tmpX|
    (y-y%3).upto(y-y%3+2) { |tmpY|
      return false if (solution[tmpY][tmpX] == num)
    }
  }
  return true
end

def solve(grid, pos=0, solution=grid.clone)
  return solution if (pos == 80)
  return solve(grid, pos+1, solution) if (solution[pos/9][pos%9] > 0)
  (1..9).select{|num| valid(num, pos%9, pos/9, solution)}.each{|num|
    solution[pos/9][pos%9] = num
    return solution if solve(grid, pos+1, solution)
  }
  solution[pos/9][pos%9] = 0
  return false
end

# Example usage and printout
grid = [
  [9,0,0,0,0,0,7,3,1],
  [6,3,1,8,7,5,0,2,0],
  [0,0,7,0,0,0,6,0,0],
  [0,0,0,3,4,7,0,1,0],
  [0,0,4,0,0,0,3,0,0],
  [0,1,0,9,8,6,0,0,0],
  [0,0,6,0,0,0,9,0,0],
  [0,4,0,7,9,8,1,6,5],
  [1,8,9,0,0,0,0,0,3]
]
solution = solve(grid)
puts solution.map{|row|row.join(",")}.join("\n")