input: Write a function to solve Any Sudoku puzzle by filling the empty cells. If a solution exists, return the solved board; otherwise, print "No solution".
  def solve_sudoku(board):
      def find_vacant_cell(board, l): 
          for row in range(9): 
              for col in range(9): 
                  if board[row][col] == 0: 
                      l[0] = row 
                      l[1] = col 
                      return True
          return False
    
      def is_safe(board, row, col, num): 
          for i in range(9): 
              if(board[row][i] == num): 
                  return False
              if(board[i][col] == num): 
                  return False
              if(board[(row//3)*3 + i//3][(col//3)*3 + i%3] == num): 
                  return False
          return True
    
      def solve(board): 
          l = [0, 0] 
          if (not find_vacant_cell(board, l)): 
              return True
          row = l[0] 
          col = l[1] 
          for num in range(1, 10): 
              if (is_safe(board, row, col, num)): 
                  board[row][col] = num 
                  if (solve(board)): 
                      return True
                  board[row][col] = 0
          return False
    
      if (solve(board)): 
          return board
      else: 
          print("No solution")
          return None
  
  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([
      [5, 3, 0, 0, 7, 0, 0, 0, 0],
      [6, 0, 0, 1, 9, 5, 0, 0, 0],
      [0, 9, 8, 0, 0, 0, 0, 6, 0],
      [8, 0, 0, 0, 6, 0, 0, 0, 3],
      [4, 0, 0, 8, 0, 3, 0, 0, 1],
      [7, 0, 0, 0, 2, 0, 0, 0, 6],
      [0, 6, 0, 0, 0, 0, 2, 8, 0],
      [0, 0, 0, 4, 1, 9, 0, 0, 5],
      [0, 0, 0, 0, 8, 0, 0, 7, 9]
  ]) == [
      [5, 3, 4, 6, 7, 8, 9, 1, 2],
      [6, 7, 2, 1, 9, 5, 3, 4, 8],
      [1, 9, 8, 3, 4, 2, 5, 6, 7],
      [8, 5, 9, 7, 6, 1, 4, 2, 3],
      [4, 2, 6, 8, 5, 3, 7, 9, 1],
      [7, 1, 3, 9, 2, 4, 8, 5, 6],
      [9, 6, 1, 5, 3, 7, 2, 8, 4],
      [2, 8, 7, 4, 1, 9, 6, 3, 5],
      [3, 4, 5, 2, 8, 6, 1, 7, 9]
  ]
output: def solve_sudoku(board):
      def find_vacant_cell(board, l): 
          for row in range(9): 
              for col in range(9): 
                  if board[row][col] == 0: 
                      l[0] = row 
                      l[1] = col 
                      return True
          return False
    
      def is_safe(board, row, col, num): 
          for i in range(9): 
              if(board[row][i] == num): 
                  return False
              if(board[i][col] == num): 
                  return False
              if(board[(row//3)*3 + i//3][(col//3)*3 + i%3] == num): 
                  return False
          return True
    
      def solve(board): 
          l = [0, 0] 
          if (not find_vacant_cell(board, l)): 
              return True
          row = l[0] 
          col = l[1] 
          for num in range(1, 10): 
              if (is_safe(board, row, col, num)): 
                  board[row][col] = num 
                  if (solve(board)): 
                      return True
                  board[row][col] = 0
          return False
    
      if (solve(board)): 
          return board
      else: 
          print("No solution")
          return None