[leetcode] N-Queens

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.."]
]

https://oj.leetcode.com/problems/n-queens/

思路：经典的8皇后问题，还是老思路，生成perm数组，perm[i]的只代表第i行放置皇后的列数，递归下去的条件是不冲突（冲突的情况：perm[j] == i || perm[j] - j == i - cur || perm[j] + j == i + cur），然后根据题目要求生成结果的形式即可。

import java.util.ArrayList;

public class Solution {
    public ArrayList<String[]> solveNQueens(int n) {
        if (n <= 0)
            return null;
        ArrayList<String[]> res = new ArrayList<String[]>();
        int[] perm = new int[n];
        slove(perm, 0, n, res);

        return res;
    }

    private void slove(int[] perm, int cur, int n, ArrayList<String[]> res) {
        if (cur == n) {
            String[] tmp = new String[n];
            for (int i = 0; i < n; i++) {
                char[] item = new char[n];
                for (int j = 0; j < n; j++)
                    item[j] = '.';
                item[perm[i]] = 'Q';
                tmp[i] = new String(item);
            }
//          System.out.println(Arrays.toString(tmp));
            res.add(tmp);

        } else {
            int i;
            for (i = 0; i < n; i++) {
                int j;
                boolean ok = true;
                for (j = 0; j < cur; j++) {
                    if (perm[j] == i || perm[j] - j == i - cur
                            || perm[j] + j == i + cur)
                        ok = false;
                }
                if (ok) {
                    perm[cur] = i;
                    slove(perm, cur + 1, n, res);
                }

            }

        }

    }

    public static void main(String[] args) {
        System.out.println(new Solution().solveNQueens(4));

    }
}