What is X-Sudoku
X-Sudoku is a Sudoku variant with the additional rule that in each of the two main diagonals each number from 1 to 9 can only appear once. The name comes from the fact that the two diagonals look like the letter X.
Because of the additional constraints there are X-Sudokus with 16 clues given and only one solution. The solution approach is nearly the same as the approach of a classic Sudoku, so it is often simpler to try to solve an X-Sudoku like a classic Sudoku and when you are stuck you can look for digits in the diagonals.