Sudoku, also known as Su Doku, is a popular form of number game. In its simplest and most common configuration, sudoku consists of a 9 × 9 grid with numbers appearing in some of the squares. The objective of the puzzle is to fill the remaining squares, using all the numbers 1–9 exactly once in each row, column, and the nine 3 × 3 subgrids. Sudoku is based entirely on logic, without any arithmetic involved.

There is a subject in computer science called 'Data Structures and Algorithms(DSA)'. Among many interesting topics of this subject, one is 'Recursion and Backtracking'. Those, who are quite familiar with backtracking, possibly have seen or solved this Sudoku question.

The algorithm to solve this puzzle goes on as described below -
* In one step we take an empty cell and put 1 to 9 in that cell. While putting we check whether that number is safe at that place (not present in same row,col etc.). If it is safe then we go on to the next empty cell there repeat the same process. In this procedure if we find any cell where nothing can be put, then we come back to the previous cell and check other possiblities there. Like this if we finally get to the last cell and fill it then sudoku is solved, or eventually if we go back to the very first empty cell and couldn't get any number that can be put there, then there is no solution.

If you want more details visit this GFG page- https://www.geeksforgeeks.org/sudoku-backtracking-7/