Understanding Sudoku Hints in 4 Steps

What Hint does in this app

• It fills the correct answer in one empty cell you select. If you have not selected a cell, or your selected cell is already filled, Hint does nothing and does not use up a hint.
• You have a limited number of uses per puzzle. The default is 3. The number on the Hint button shows how many you have left (the keyboard shortcut is H).
• It deducts 100 points from your score. A wrong number, counted under Mistakes, costs 50 points, so a hint costs more than a mistake. In return, a hint is never wrong.
• If “Auto-remove notes” is enabled (the default), a number filled by Hint is also removed from the notes in the same row, column and 3×3 box.

A hint tells you the answer, but not the reason. After using one, work backward to find the reason so that you can spot the same pattern yourself next time. The four steps below show you how.

Four steps

1. Select a cell you are stuck on and use Hint. Hint only fills the selected cell, so choose one that will help you most with your next move. A cell in a row or column with few empty cells is a good choice.
2. Check from the cell's perspective. Set aside the revealed number and check which numbers from 1 to 9 are in its row, column and box. If all eight other numbers are already there, the cell was a Naked Single.
3. If that does not explain it, check from the number's perspective. If step 2 does not explain the answer, look at why the revealed number cannot go in the other empty cells in the same row, column or box. If every other empty cell is ruled out, it was a Hidden Single in that row, column or box.
4. Name the method and reflect. If step 2 explained it, you could have found it with “Finding your first number ①” in the rules article. If step 3 explained it, you could have used “Finding your first number ②” or step 2 of Stuck on Sudoku? A 7-Step Checklist. Recognizing the method helps you find it without a hint in your next puzzle.

Example — a hint explained from the cell's perspective

Suppose you select r2c7 in the example below and use Hint, which fills in 5.

c1c2c3c4c5c6c7c8c9

r1r2r3r4r5r6r7r8r9

3

1

6

9

2

4

6

5

7

9

8

3

3

4

5

2

6

3

9

8

2

3

8

7

5

4

2

5

7

1

2

8

3

Follow step 2 and check from the cell's perspective.

• Row 2 contains 2, 4, 6 and 7.
• Column 7 contains 8, 1 and 3.
• Box 3 contains 6, 9, 7, 8 and 3.

Together, these three areas contain all eight numbers 1, 2, 3, 4, 6, 7, 8 and 9. Only 5 remains. This hint therefore filled a cell you could have solved with “Finding your first number ①.” Next time you get stuck, start by checking all three areas for one empty cell like this.

When these steps do not explain the hint

• Neither step 2 nor step 3 may explain the revealed number. That cell requires a more complex relationship between candidates, such as a Naked Pair, or several steps of reasoning. Rather than spending too long searching for the reason, look for nearby cells you can now solve using the revealed number.
• A hint guarantees the answer, but that does not mean the selected cell is the easiest next move on the board. What you gain depends on the cell you choose in step 1.

Common mistakes

• You press Hint without selecting a cell. In this app, nothing happens and no hint is used up. Select an empty cell first.
• You use a hint and move straight to the next cell. Without checking the reason, you may get stuck at the same point in your next puzzle.
• You guess to save a hint. A wrong guess increases your mistake count, and using Undo does not reduce it. When you are unsure, a hint is safer than a guess.

Practice puzzle

On the same example board, suppose you now use Hint on r3c5 and it fills in 2. Looking at r3c5 alone leaves several candidates, so step 2 cannot explain the answer.

c1c2c3c4c5c6c7c8c9

r1r2r3r4r5r6r7r8r9

3

1

6

9

2

4

6

7

2

9

8

3

3

4

5

2

6

3

9

8

2

3

8

7

5

4

2

5

7

1

2

8

3

Use step 3, the number's perspective, to explain why r3c5 is 2. Look at why 2 cannot go in the other empty cells in row 3.

Show answer

Row 3 has six empty cells: r3c1, r3c2, r3c3, r3c4, r3c5 and r3c9. r3c1, r3c2 and r3c3 cannot be 2 because box 1 already has a 2 at r2c2. r3c4 cannot be 2 because column 4 has a 2 at r9c4, and r3c9 cannot be 2 because column 9 has a 2 at r7c9. Only r3c5 can hold row 3's 2: a cell you could have solved with “Finding your first number ②.”