<p>The <strong>n-queens</strong> puzzle is the problem of placing <code>n</code> queens on an <code>n x n</code> chessboard such that no two queens attack each other.</p>

<p>Given an integer <code>n</code>, return <em>all distinct solutions to the <strong>n-queens puzzle</strong></em>. You may return the answer in <strong>any order</strong>.</p>

<p>Each solution contains a distinct board configuration of the n-queens&#39; placement, where <code>&#39;Q&#39;</code> and <code>&#39;.&#39;</code> both indicate a queen and an empty space, respectively.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2020/11/13/queens.jpg" style="width: 600px; height: 268px;" />
<pre>
<strong>Input:</strong> n = 4
<strong>Output:</strong> [[&quot;.Q..&quot;,&quot;...Q&quot;,&quot;Q...&quot;,&quot;..Q.&quot;],[&quot;..Q.&quot;,&quot;Q...&quot;,&quot;...Q&quot;,&quot;.Q..&quot;]]
<strong>Explanation:</strong> There exist two distinct solutions to the 4-queens puzzle as shown above
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> n = 1
<strong>Output:</strong> [[&quot;Q&quot;]]
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= n &lt;= 9</code></li>
</ul>

class Solution {
public:
    vector<vector<string>> solveNQueens(int n) {
        vector<vector<string>> res;
        vector<string> sub(n, string(n, '.'));
        helper(n, 0, sub, {}, {}, {}, res);
        return res;
    }

private:
    void helper(int n, int curr, vector<string> sub, unordered_set<int> cols,
                unordered_set<int> diag1, unordered_set<int> diag2,
                vector<vector<string>>& res) {

        if (curr == n) {
            res.push_back(sub);
            return;
        }

        for (int j = 0; j < n; ++j) {
            if (cols.contains(j) || diag1.contains(curr-j) || diag2.contains(curr+j)) continue;

            sub[curr][j] = 'Q';
            cols.insert(j); diag1.insert(curr-j); diag2.insert(curr+j);

            helper(n, curr + 1, sub, cols, diag1, diag2, res);

            sub[curr][j] = '.';
            cols.erase(j); diag1.erase(curr-j); diag2.erase(curr+j);
        }
    }
};
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