Sudoku solving algorithm C++

Question

I\'m trying to make a Sudoku Solving program for a couple of days but I\'m stuck with the methods. I found this algorithm here but I don\'t really understand it:

Answer 1

There is one very important modification to recursive algorithms: Use most constrained first approach. This means first to solve a cell with smallest number of possible candidates (when direct row/column/block conflicts are removed).

Another modification is: Change the board in-place; do not copy it. In each recursive call you modify only one cell on the board, and that cell used to be empty. If that call doesn't end up in a solved board somewhere down the recursive call tree, just clear the cell again before returning - this returns the board into original state.

You can find a very short and fast solution in C# on address: Sudoku Solver. It solves arbitrary sudoku board in about 100 steps only, all thanks to the most constrained first heuristic.