input: def solve_sudoku(board):
      """
      Solve a Sudoku puzzle using backtracking.
      >>> board = [
      ...     [7,8,0,4,0,0,1,2,0],
      ...     [6,0,0,0,7,5,0,0,9],
      ...     [0,0,0,6,0,1,0,7,8],
      ...     [0,0,7,0,4,0,2,6,0],
      ...     [0,0,1,0,5,0,9,3,0],
      ...     [9,0,4,0,6,0,0,0,5],
      ...     [0,7,0,3,0,0,0,1,2],
      ...     [1,2,0,0,0,7,4,0,0],
      ...     [0,4,9,2,0,6,0,0,7]
      ... ]
      >>> solve_sudoku(board)
      [7, 8, 5, 4, 3, 9, 1, 2, 6]
      [6, 1, 2, 8, 7, 5, 3, 4, 9]
      [4, 9, 3, 6, 2, 1, 5, 7, 8]
      [8, 5, 7, 9, 4, 3, 2, 6, 1]
      [2, 6, 1, 7, 5, 8, 9, 3, 4]
      [9, 3, 4, 1, 6, 2, 7, 8, 5]
      [5, 7, 8, 3, 9, 4, 6, 1, 2]
      [1, 2, 6, 5, 8, 7, 4, 9, 3]
      [3, 4, 9, 2, 1, 6, 8, 5, 7]
      """
output:     def is_valid(board, row, col, num):
          for i in range(9):
              if board[row][i] == num or board[i][col] == num or board[row//3*3 + i//3][col//3*3 + i%3] == num:
                  return False
          return True
  
      def solve(board):
          for row in range(9):
              for col in range(9):
                  if board[row][col] == 0:
                      for num in range(1, 10):
                          if is_valid(board, row, col, num):
                              board[row][col] = num
                              if solve(board):
                                  return True
                              board[row][col] = 0
                      return False
          return True
  
      solve(board)
      return board