Logic Masters Deutschland e.V.

If you enjoy this type of puzzle with only ?? clues -- you might consider trying X-Sums and Sandwiches

The Rules:

1. [Sudoku]: Normal sudoku rules apply. Fill every row, column, and 3x3 box with the digits from 1 to 9 once each.
2. Clues outside the grid are simultaneously sandwich clues, and skyscraper sum clues (i.e. if the sandwich clue is 13, then the skyscraper sum clue must be 13) A single question mark must be a one digit number and a double question mark must be two digit number (no leading zeros).
   • [Sandwich]: The sum of the digits between the 1 and the 9 in that row or column.
   • [Skyscraper Sums]: The sum of the digits visible in that row or column from the direction of the clue, where higher digits block the view of smaller ones.
     e.g. 314592687 would have a skyscraper sum clue of 3+4+5+9 = 21 on the left, or 7+8+9 = 24 on the right.

The Puzzle:

Try it on Sven's Sudokupad or this one with room outside the grid for note taking