N-Queens

Link: http://oj.leetcode.com/problems/n-queens/

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  ArrayList<String[]> result = new ArrayList<String[]>();

    public  ArrayList<String[]> solveNQueens(int n) {
        if (n < 1)
            return null;
        result = new ArrayList<String[]>();
        int[] queens = new int[n + 1];
        for (int i = 1; i <= n; i++) {
            queens[1] = i;
            dfs(2, n, queens);
        }
        return result;
    }

    public  void dfs(int row, int size, int[] queens) {
        if (row > size) {
            print_queen(size,queens);
            return;
        }
        for (int i = 1; i <= size; i++) {
            if (isValid(row, i, queens)) {
                queens[row] = i;
                dfs(row + 1, size, queens);
                queens[row] = 0;

            }
        }
    }

    public  boolean isValid(int row, int col, int[] queens) {
        for (int i = 1; i < row; i++) {
            if (queens[i] != 0
                    && (queens[i] == col || (Math.abs(i - row) == Math
                            .abs(queens[i] - col)))) {
                return false;
            }
        }

        return true;
    }
        public  void print_queen(int size,int[] queens) {
        String[] str = new String[size];
        for (int i = 1; i <=size; i++) {
            int temp = queens[i];
            String s = "";
            for (int j = 0; j < size; j++) {
                if (j + 1 == temp) {
                    s += "Q";
                    continue;
                }
                s += ".";
            }
            str[i - 1] = s;
        }
        result.add(str);
    }
}