public static SudokuGrid RecursiveSolve(SudokuGrid gridToSolve)
{
if (!gridToSolve.IsValid)
{
return null;
}
if (gridToSolve.IsComplete)
{
return gridToSolve;
}
// So we have a grid that's incomplete and might have solutions, so:
var nextPosTuple = gridToSolve.NextPositionToPopulate();
foreach (var possibleEntry in nextPosTuple.Item3)
{
var sudokuGridToAmend = new char[9, 9];
Array.Copy(gridToSolve.sudokuArray, sudokuGridToAmend, 81);
sudokuGridToAmend[nextPosTuple.Item1, nextPosTuple.Item2] = possibleEntry;
// Recursion is magic!
var newGridToSolve = new SudokuGrid(sudokuGridToAmend);
var solvedGrid = RecursiveSolve(newGridToSolve);
if (solvedGrid != null)
{
return solvedGrid;
}
}
// Otherwise the grid we've been given has no solutions, so
return null;
}