<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:
    unordered_set<int> c, ld, rd;
    vector<vector<string>> r;
    void h(vector<string> &b, int n, int row) {
        if (row==n) {
            r.push_back(b);
            return;
        }
        for (int j=0; j<n; j++) {
            auto x=c.insert(j);
            auto y=ld.insert(row-j);
            auto z=rd.insert(row+j);
            if (x.second && y.second && z.second) {
                b[row][j]='Q';
                h(b, n, row+1);
                c.erase(j);
                ld.erase(row-j);
                rd.erase(row+j);
                b[row][j]='.';
            }
            else {
                if (x.second) c.erase(j);
                if (y.second) ld.erase(row-j);
                if (z.second) rd.erase(row+j);
            }
        }
    }
    vector<vector<string>> solveNQueens(int n) {
        ios_base::sync_with_stdio(false);
        cin.tie(NULL);
        cout.tie(NULL);
        vector<string> b(n, string(n, '.'));
        h(b, n, 0);
        return r;
    }
};
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