Sudoku

Goal

Fill every empty square with a number from 1 to 9. Each column, each row and each 3×3 box marked by the thick lines may hold a number only once.

How to play

Use the starting numbers as clues to work out what belongs in the empty squares. Select a square, then tap a number below the grid to enter it. The starting numbers cannot be changed. Before entering a number, check that its row, column and 3×3 box do not already contain that number. Use "Undo" to undo your last action.

"Moves" increases by one each time you enter, erase or replace a number or turn a note on or off, and "Undo" reduces the count by one.

Tactics

Start with nearly complete rows, columns or boxes

Look first for rows, columns and 3×3 boxes with the fewest empty squares. Check which numbers from 1 to 9 are already there and list the missing ones. If only one square is empty, the missing number belongs there. After entering a number, check its row and column again.

Narrow down the candidates for one square

Pick an empty square and rule out every number already in its row, column and box. If only one number remains, it belongs there. If several candidates remain, do not guess — check another square. You cannot decide whether a number fits just by looking at how many squares are empty.

Find where a particular number can go

Next, choose a box and find where a particular number can go. If 5 is missing, use the 5s in other rows and columns to rule out squares. If only one possible square remains, that is where 5 goes. Each number you enter changes the possibilities, so go back and check the squares you could not solve earlier.