Solving techniques
In brief
You don't guess: for each empty cell, you look for the numbers that are still possible, the candidates. A cell with only one candidate left, or a number with only one place left in its row, column or box, can be filled in with certainty. These two rules, the singles, are enough for all the Easy, Medium and Hard grids in the app.
Candidates and notes
The candidates of a cell are the numbers missing from its row, its column and its box. In the app, the Notes button lets you write them small in the cell. Below, Medium grid no. 8 of the app at the start, with the candidates of a few cells in the first row.
| 7 | 3 | 5 | 13 | 2 | 4 | 8 | 6 | empty |
| empty | empty | 9 | empty | empty | 5 | 2 | empty | empty |
| 4 | empty | 1 | empty | 9 | 6 | empty | empty | 5 |
| empty | 5 | empty | 2 | empty | 9 | empty | empty | 6 |
| empty | 9 | 7 | empty | empty | empty | 5 | empty | empty |
| 2 | 6 | empty | empty | 5 | 8 | 9 | empty | 1 |
| empty | 4 | empty | 6 | empty | empty | 1 | 5 | 7 |
| empty | 1 | 6 | 9 | empty | 7 | empty | empty | 8 |
| 8 | 7 | empty | empty | empty | empty | empty | 9 | empty |
The naked single
In row 1, column 2 (boxed cell), all the numbers but one are already present in the row, column or box: the only candidate is 3. Write it in without hesitation.
The hidden single
The cell in row 1, column 4 has 2 candidates: looked at alone, it can't be decided. But in row 1, the number 1 has only one possible place left: this cell. So it goes there. This is how you search, unit by unit, for the number that has only one place.
The naked pair
When singletons run out, look at the notes. If two cells in the same unit have only the same two candidates, those two digits must go in those two cells, so you can cross them out of the notes of the other cells in the unit.
Example on Inhuman grid #33 in the app: after 21 singletons, 40 cells are still empty and no singleton is left. In box 1, the cells in row 2, column 1 and in row 3, column 1 have only the candidates 3 and 5. Cross these digits out of the other cells in the box (3 fewer notes), and a new singleton appears. After that, singletons are enough up to the last cell.
| 9 | 4 | 1 | 3 | empty | empty | 8 | 2 | 5 |
| 35 | 2 | 8 | 4 | empty | empty | empty | empty | empty |
| 35 | 367 | 3567 | 1 | 2 | 8 | 4 | empty | empty |
| 2 | empty | empty | empty | empty | empty | empty | 4 | 3 |
| 4 | empty | 9 | 5 | empty | 3 | 6 | 7 | 2 |
| 7 | empty | empty | 2 | empty | 4 | empty | empty | empty |
| 1 | 9 | 2 | 8 | 3 | empty | 7 | empty | 4 |
| 8 | 5 | empty | empty | 4 | 2 | empty | empty | empty |
| 6 | empty | 4 | empty | empty | empty | 2 | empty | empty |
And beyond
Of the 100 Inhuman grids, 79 can be finished with singletons alone and 87 by adding naked pairs. The others call for more advanced techniques, still without guessing, since each grid has only one solution.