N-Queens II

public class Solution {
    public int totalNQueens(int n) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        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.size();
    }
    
    private 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);
            }
        }

    }
    private 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;
    }
}