java使用回溯法求解n皇后问题

以下是Java使用回溯法求解N皇后问题的代码示例：

public class NQueens {
    private int n;
    private int[] queens;
    private boolean[] colUsed;
    private boolean[] diagonal1Used;
    private boolean[] diagonal2Used;
    private List<List<String>> res;

    public List<List<String>> solveNQueens(int n) {
        this.n = n;
        queens = new int[n];
        colUsed = new boolean[n];
        diagonal1Used = new boolean[2 * n - 1];
        diagonal2Used = new boolean[2 * n - 1];
        res = new ArrayList<>();
        dfs(0);
        return res;
    }

    private void dfs(int row) {
        if (row == n) {
            res.add(generateBoard());
            return;
        }
        for (int col = 0; col < n; col++) {
            int diagonal1 = row + col;
            int diagonal2 = n - 1 - row + col;
            if (colUsed[col] || diagonal1Used[diagonal1] || diagonal2Used[diagonal2]) {
                continue;
            }
            queens[row] = col;
            colUsed[col] = true;
            diagonal1Used[diagonal1] = true;
            diagonal2Used[diagonal2] = true;
            dfs(row + 1);
            queens[row] = 0;
            colUsed[col] = false;
            diagonal1Used[diagonal1] = false;
            diagonal2Used[diagonal2] = false;
        }
    }

    private List<String> generateBoard() {
        List<String> board = new ArrayList<>();
        for (int i = 0; i < n; i++) {
            char[] row = new char[n];
            Arrays.fill(row, '.');
            row[queens[i]] = 'Q';
            board.add(new String(row));        }
        return board;
    }
}