[C] Convert uppercase to lowercase

A funtion to convert a char to its lowercase equivalent, if it has a lowercase equivalent else return 0, without using the funtion tolower() from ctype.h

[Project] Sudoku Solver

Sudoku is a puzzle game played on a 9×9 grid, subdivided into 3×3 sub-grids called “regions”.
The objective of the game is to fill in the blank cells with the numbers 1 through 9, such that each number appears exactely once in each row, column and region.