input: Write a function to solve a Sudoku puzzle using the given board as input. The function should be capable of solving any valid Sudoku puzzle. If a valid solution exists, it should print the solved board; otherwise, it should indicate that no solution exists.
  assert solve_sudoku([[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]]) == [[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]]
  assert solve_sudoku([[0,0,0,4,0,0,1,2,0],[0,0,0,0,0,5,0,0,9],[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]]) == [[3,6,5,4,8,9,1,2,7],[7,1,2,8,3,5,6,4,9],[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]]
  assert solve_sudoku([[1,2,3,4,5,6,7,8,9],[0,0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0,0]]) == [[1,2,3,4,5,6,7,8,9],[4,5,6,7,8,9,1,2,3],[7,8,9,1,2,3,4,5,6],[2,1,4,3,6,5,8,9,7],[3,6,5,8,9,7,2,1,4],[8,9,7,2,1,4,3,6,5],[5,3,1,6,4,2,9,7,8],[6,4,2,9,7,8,5,3,1],[9,7,8,5,3,1,6,4,2]]
  assert solve_sudoku([[0,0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,0,0]]) == [[1,2,3,4,5,6,7,8,9],[4,5,6,7,8,9,1,2,3],[7,8,9,1,2,3,4,5,6],[2,1,4,3,6,5,8,9,7],[3,6,5,8,9,7,2,1,4],[8,9,7,2,1,4,3,6,5],[5,3,1,6,4,2,9,7,8],[6,4,2,9,7,8,5,3,1],[9,7,8,5,3,1,6,4,2]]
output: def solve_sudoku(board):
      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
  
      if solve(board):
          return board
      else:
          return None