This is my own algorithm to resolve a Sudoku 9 x 9 cells, this is the logic of the resolution:

first I have a fixed cells with some values , for example this;

the first approach to resolve the Sudoku is to start at the first blank cell in this case x=3 y=1

Fist I try to search some possible value (from 1 to 9 ) checking if this value is possible (not is on this 3 rectangles):

Then cheeking recursively the possible values of the rest of the cells, is not have solution then the program take step back to check another

You can download the code at Github:

If you want only de app compiled:

this is the sample video resolution, first, enter the fixed values and then calculate: