All Sudoku Variants
45 cells (right triangle) · Shape Variant

Sujiken

Also known as Diagonal Triangle Sudoku

Half a Sudoku, cut along the diagonal: 45 square cells in a right triangle where no digit repeats in any row, column, diagonal, or bold triangular region.

5 3 7 9 1

An example Sujiken: only the 45-cell right triangle (white) is the puzzle; digits never repeat in any row, column or diagonal.

Sujiken — from the Japanese sujikai, "diagonal" — is Sudoku cut in half along the diagonal, leaving a right triangle of 45 square cells. You place the digits 1–9 so that none repeats in any row, column, diagonal, or bold triangular region. The diagonal rule is what gives it its edge.

The rule: a triangle with diagonals

Fill the triangle so every row and column holds its digits without repeats, exactly as in Sudoku. On top of that, no diagonal line may repeat a digit either, and the grid's three bold triangular regions each stay repeat-free. The greyed cells above the diagonal are not part of the puzzle.

Solving strategy

  • Work the long diagonal. The main diagonal is the triangle's longest line and a rich source of eliminations.
  • Lean on the short rows. Rows near the tip have very few cells, so they resolve quickly and seed the rest.
  • Check three ways. Every placement must clear its row, its column and every diagonal it lies on.

Common mistakes

  • Ignoring the diagonals. They are a full constraint here, not decoration.
  • Filling the greyed cells. Only the 45-cell triangle is live.
  • Forgetting the triangular regions. The three bold regions each need their digits without repeats.

Can I play it here?

Not yet — Sujiken isn't one of the puzzle types SudokuStreak generates for now. But you can learn its rules above, and there's plenty here to sink your teeth into meanwhile: