Sudoku Solver by Recursive Backtracking

A standard implementation of recursive backtracking in solving a Sudoku puzzle

Recursive backtracking is a well-known brute-force search algorithm. As long as a Sudoku puzzle is valid, it can be solved by recursive backtracking, i.e. trying all possible numbers systematically. When the puzzle has many solutions, it will output the lexicographically first one. If it has no solution, the output is the original unsolved puzzle.

*Thumbnail image taken from Wikipedia: https://en.wikipedia.org/wiki/Sudoku_solving_algorithms