<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>> ans;
    void solve(int i,vector<string> tmp,unordered_set<int> col,unordered_set<int> pos,unordered_set<int> neg){
        int n = tmp.size();
        if(i==n){
            ans.push_back(tmp);
            return;
        }

        for(int j=0;j<n;j++){

            if(col.contains(j) || pos.contains(i+j) || neg.contains(i-j)) continue;
            col.insert(j);
            pos.insert(i+j);
            neg.insert(i-j);
            tmp[i][j]='Q';

            solve(i+1,tmp,col,pos,neg);

            col.erase(j);
            pos.erase(i+j);
            neg.erase(i-j);
            tmp[i][j]='.';
        }
    }
    vector<vector<string>> solveNQueens(int n) {
        
        vector<string>vec(n,string(n,'.'));
        unordered_set<int>col,pos,neg;
        solve(0,vec,col,pos,neg);
        return ans;
    }
};

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