// Solve algorithm
func (s *Sudoku) Solve() bool {
	// Find the next pending position in the board
 	pending, row, col := s.pendingPositions()
	// If there aren't more pending positions we did
	// and the problem is solved
	if !pending {
		return true
	}
	// If not, let's try to find a solution for the pending position
	// in row and col. We will try with digits from 1 to 9
	for i := 1; i <= 9; i++ {
		// Check is the current digit is valid for the given position
		// This function checks row, column and box validity
		if s.valid(i, row, col) {
		// If is valid assign this value to the given position
		s.grid[row][col] = i
		// Recursive call to Solve method. If the Sudoku
		// is done return  true
		if s.Solve() {
			return true
		}
		// If the current value does not fixed the Sudoku we
		// undo the changes (Backtracking comes to scene!!!)
		// and we try with the next one
		s.grid[row][col] = PENDING
		}
	}
	// We don't find any solution for the Sudoku problem
	return false
}