Sudoku
Nine rows, nine columns and nine 3×3 boxes, each holding 1 to 9 exactly once. A new puzzle is built every time you open the page, and only puzzles with a single solution are handed out. Pick a square and tap a number, or type 1-9.
Puzzles are made by erasing a finished grid
A complete solution is filled in at random first, then the 81 cells are visited in shuffled order and erased one at a time. A cell stays empty only if the puzzle still has exactly one solution and does not exceed the chosen difficulty; otherwise it is put back. After the pass the difficulty is measured again, and if it falls short the whole thing is rebuilt (at most twenty times).
Hard means pencil marks and pairs
Easy is finished by counting the candidates of each empty cell and filling the ones with a single candidate. Normal takes one digit at a time and asks where in this box it can go — selecting a cell highlights every cell holding the same digit. Hard needs pencil marks. When a digit's candidates inside a box all sit on one line, that digit can be removed from the rest of the line in other boxes; when two cells share the same two candidates, those two digits go from the other cells of that unit. Notes are cleared by hand only — placing a digit does not remove it from neighbouring cells' notes.
The clock starts the moment the puzzle appears
It starts when the grid is drawn, not at your first entry. A record is set only when the grid is full with no conflicts, compared against that level's record and kept only if shorter, and it stays in this browser. Today's board is counted separately — the seed comes from the date, so everyone gets the same normal-level puzzle, and only the first finish that day is kept. Boards change at midnight Korean time.
Common questions
QCould a puzzle have more than one solution?
No. Every time a square is emptied, the puzzle is checked for still having exactly one solution; if a second appears, the square is filled back in. A puzzle with two solutions tells people they are wrong when they are not. Forty puzzles at each difficulty were generated and all had exactly one solution.
QHow is difficulty decided?
By which techniques a solve actually needs. Easy falls to "only one number fits this square"; medium needs "this number fits only one square in the row"; hard needs "it is somewhere along this line even though we cannot say where". All 120 generated puzzles landed on exactly their own grade.
QDoesn't a puzzle with fewer givens mean a harder puzzle?
No, which is why givens are not used for grading. Measured, easy puzzles averaged 25.7 givens and hard ones 24.2 — nearly the same — and 25 of 30 easy puzzles fell inside the range of given counts seen in hard ones.
QWhat are notes for?
Pencilling in the candidates a square could hold. Hard puzzles are difficult to carry in your head without them. With notes on, tapping a number writes it small into the square, and tapping again removes it. A number repeated in its row, column or box turns red, so a wrong entry shows immediately.
QWhy is there no hint button?
A hint that fills a square skips the part where you work out where to look, which is the thing sudoku teaches. Instead, a number repeated in its row, column or box turns red, so a mistake shows immediately.
QIf I refresh, is my puzzle still there?
No. The puzzle is generated after the page loads, so a refresh brings a new puzzle and the clock starts again from zero. What survives is the best time per level and today's board result.
QIf a digit does not turn red, is it correct?
Not necessarily. Red appears only when a digit clashes with one already placed. A wrong digit that conflicts with a cell still empty is not marked; the clash appears later when that cell is filled, and the screen does not say which of the two is wrong.