Solution.cs

public class Solution {
    private int n;
    private int[] col;
    private int[] dg;
    private int[] udg;
    private IList<IList<string>> ans = new List<IList<string>>();
    private IList<string> t = new List<string>();

    public IList<IList<string>> SolveNQueens(int n) {
        this.n = n;
        col = new int[n];
        dg = new int[n << 1];
        udg = new int[n << 1];
        dfs(0);
        return ans;
    }

    private void dfs(int i) {
        if (i == n) {
            ans.Add(new List<string>(t));
            return;
        }
        for (int j = 0; j < n; ++j) {
            if (col[j] + dg[i + j] + udg[n - i + j] == 0) {
                char[] row = new char[n];
                Array.Fill(row, '.');
                row[j] = 'Q';
                t.Add(new string(row));
                col[j] = dg[i + j] = udg[n - i + j] = 1;
                dfs(i + 1);
                col[j] = dg[i + j] = udg[n - i + j] = 0;
                t.RemoveAt(t.Count - 1);
            }
        }
    }
}