public void SolveSudoku(char[][] board)

{

int[] cols = new int[9];

int[] rows = new int[9];

int[] squares = new int[9];

for (int r = 0; r < 9; r++)

{

for (int c = 0; c < 9; c++)

{

if (board[r][c] == '.') continue;

int val = board[r][c] - '1';

rows[r] |= (1 << val);

cols[c] |= (1 << val);

squares[(r / 3) * 3 + (c / 3)] |= (1 << val);

}

}

List<(int r, int c)> fillList = [];

for (int i = 0; i < 9; i++)

{

for (int j = 0; j < 9; j++)

{

if (board[i][j] == '.') fillList.Add((i, j));

}

}

bool isFinish = false;

void BackTracking(int index)

{

if (fillList.Count == 0 || index >= fillList.Count)

{

isFinish = true;

return;

}

var (r, c) = fillList[index];

for (int val = 0; val < 9; val++)

{

if ((rows[r] & (1 << val)) > 0 || (cols[c] & (1 << val)) > 0 || (squares[(r / 3) * 3 + (c / 3)] & (1 << val)) > 0)

{

continue;

}

board[r][c] = (char)(val + '1');

rows[r] |= (1 << val);

cols[c] |= (1 << val);

squares[(r / 3) * 3 + (c / 3)] |= (1 << val);

BackTracking(index + 1);

if (isFinish) return;

// rollback

board[r][c] = '.';

rows[r] &= ~(1 << val);

cols[c] &= ~(1 << val);

squares[(r / 3) * 3 + (c / 3)] &= ~(1 << val);

}

}

BackTracking(0);

}