Solution.java

class Solution {
    private boolean ok;
    private char[][] board;
    private List<Integer> t = new ArrayList<>();
    private boolean[][] row = new boolean[9][9];
    private boolean[][] col = new boolean[9][9];
    private boolean[][][] block = new boolean[3][3][9];

    public void solveSudoku(char[][] board) {
        this.board = board;
        for (int i = 0; i < 9; ++i) {
            for (int j = 0; j < 9; ++j) {
                if (board[i][j] == '.') {
                    t.add(i * 9 + j);
                } else {
                    int v = board[i][j] - '1';
                    row[i][v] = col[j][v] = block[i / 3][j / 3][v] = true;
                }
            }
        }
        dfs(0);
    }

    private void dfs(int k) {
        if (k == t.size()) {
            ok = true;
            return;
        }
        int i = t.get(k) / 9, j = t.get(k) % 9;
        for (int v = 0; v < 9; ++v) {
            if (!row[i][v] && !col[j][v] && !block[i / 3][j / 3][v]) {
                row[i][v] = col[j][v] = block[i / 3][j / 3][v] = true;
                board[i][j] = (char) (v + '1');
                dfs(k + 1);
                row[i][v] = col[j][v] = block[i / 3][j / 3][v] = false;
            }
            if (ok) {
                return;
            }
        }
    }
}