N皇后问题

The n-queens puzzle is the problem of placing n queens on an n×n chessboard such that no two queens attack each other.

Given an integer n, return all distinct solutions to the n-queens puzzle.

Each solution contains a distinct board configuration of the n-queens' placement, where 'Q' and '.' both indicate a queen and an empty space respectively.

For example,
There exist two distinct solutions to the 4-queens puzzle:

[
 [".Q..",  // Solution 1
  "...Q",
  "Q...",
  "..Q."],

 ["..Q.",  // Solution 2
  "Q...",
  "...Q",
  ".Q.."]
]
boolean canPlace(int row, int k, int n) {
        boolean b = true;
        for (int i = 1; i <= n; i++) {
            if (c[i] != 0 && Math.abs(row - c[i]) == Math.abs(k - i)) {
                b = false;
                break;
            }
        }
        return c[k] == 0 && b;
    }

    int c[];
    List<String[]> list = new ArrayList<String[]>();

    void queen(int row, int n) {
        if (row > n) {
            res= getRes(n);
            list.add(res);
            res = init(n);
        } else {
            for (int k = 1; k <= n; k++) { //试探第row行每一个列
                if (canPlace(row, k, n)) {
                    c[k] = row;  //放置皇后
                    queen(row + 1, n);  //继续探测下一行
                    c[k] = 0;
                }
            }
        }
    }

    String[] getRes(int n) {
        String res[] = new String[n];
        for (int i = 0; i < n; i++) {
            res[i] = "";
        }
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < n; j++) {
                if (i + 1 == c[j + 1])
                    res[i] += "Q";
                else
                    res[i] += ".";
            }

        }
        return res;
    }

    String[] init(int n) {
        String res[] = new String[n];
        for (int i = 0; i < n; i++) {
            res[i] = "";

        }
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < n; j++) {
                res[i] += ".";
            }

        }
        return res;
    }