See https://www.sudokuwiki.org/Getting_Started
Notice how you cannot fit two of the same number in a group.
So a cell cannot be a digit if it sees that digit somewhere else.
If a cell sees every digit except one, that leftover digit fills the cell!
Here's an example with the first sudoku:
Notice the cell in row 7, column 9 (aka R7C9)
Somehow the cell sees every digit that's not 4.
So the only possible digit [aka candidate] for this cell is 4!
Here are all the digits R7C9 sees:
Row: 6, 2, 8
Column: 3, 1, 6, 5, 9
Box: 2, 8, 5, 7, 9
This is probably the easiest strategy. When a row, column, or box is almost filled just fill in the remaining digit.
Todo: Add pictures
This is covered by Last possible digit in cell
If there's only one space left for a digit in a group, it can be filled in (just like Last possible digit in cell).
Now in (image down), the lower left box (box 7) has only one space left for 8!
So now 8 can be placed there.
This happens with:
columns 2 and 5
rows 6 and 8
boxes 2 (top middle), 6 (middle right), and 7
In the second picture, I've placed yellow squares for all the empty cells that see the given eights
"Check for solved" counts the number of solved digits. It checks if the sudoku is already solved. (Also checks for invalid sudokus)
"Update candidates" = Last possible digit in cell
A "candidate" is a possibility of a cell. If there are new solved cells, the candidates are updated
"Hidden singles" = Last place for a digit in a group
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