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.."]
]

public class Solution {
    public static void main(String[] args) {
        ArrayList<String[]> result = solveNQueens(4);
        System.out.println(result.size());
        for(int i = 0; i < result.size(); i++){
            System.out.println("solution:" + i);
            for(int j = 0; j < result.get(i).length; j++){
                System.out.println(result.get(i)[j]);
            }
        }
    }

    public static ArrayList<String[]> solveNQueens(int n) {
        // Start typing your Java solution below
        // DO NOT write main() function
        ArrayList<String[]> result = new ArrayList<String[]>();
        int depth = 0;
        int[] rows = new int[n];
        for (int i = 0; i < n; i++) {
            rows[i] = n + 1;
        }
        dfs(depth, rows, n, result);
        return result;
    }

    public static void dfs(int depth, int[] rows, int n, ArrayList<String[]> result) {
        if (depth == n) {
            String[] s = new String[n];
            for (int i = 0; i < n; i++) {
                int m = rows[i];
                StringBuilder tmp = new StringBuilder();
                for (int j = 0; j < n; j++) {
                    if (j == m) {
                        tmp.append("Q");
                        continue;
                    }
                    tmp.append(".");
                }
                s[i] = tmp.toString();
            }
            result.add(s);
            return;
        }
        for (int i = 0; i < n; i++) {
            rows[depth] = i;
            if (isValid(rows, depth)) {
                dfs(depth + 1, rows, n, result);
            }
        }

    }
    public static boolean isValid(int[] rows, int depth) {
        for (int i = 0; i < depth; i++) {
            if (rows[i] == rows[depth]
                    || Math.abs(rows[i] - rows[depth]) == Math.abs(i - depth)) {
                return false;
            }
        }
        return true;
    }
}