private static byte[] Solve(Sudoku sudoku)
{
bool updated;
bool solved;
do
{
updated = false;
solved = true;
for (int i = 0; i < sudoku.Answer.Length; i++)
{
if (sudoku.Answer[i] != 0)
{
continue;
}
solved = false;
int c = sudoku.GetCandidateBit(i);
if (c == 0)
{
return(null);
}
int lowest = LowestOneBit(c);
if (c == lowest)
{
sudoku.Update(i, ToNum(lowest));
updated = true;
}
}
} while (updated);
if (solved)
{
return(sudoku.Answer);
}
// 候補が少ない箇所を探す
int count = int.MaxValue;
int index = 0;
for (int i = 0; i < sudoku.Answer.Length; i++)
{
if (sudoku.Answer[i] != 0)
{
continue;
}
int c = BitCount(sudoku.GetCandidateBit(i));
if (c < count)
{
count = c;
index = i;
}
}
// 複数ある候補のうちの一つを仮定して、再帰的に解析
int candidate = sudoku.GetCandidateBit(index);
for (int i = 0; i < 9; i++)
{
int bit = (candidate & (1 << i));
if (bit == 0)
{
continue;
}
Sudoku newSudoku = new Sudoku(sudoku);
newSudoku.Update(index, (byte)(i + 1));
byte[] answer = Solve(newSudoku);
if (answer != null)
{
return(answer);
}
}
return(null);
}