<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:
    void recur(int ind , int n , vector<string> board,
    set<int> &cols,set<int> &posd,set<int> &negd,
    vector<vector<string>> &res){
        if( ind ==n){
            res.push_back(board);
            return;
        }

        for(int j=0;j<n;j++){
            if( cols.find(j) != cols.end() || posd.find(j+ind) != posd.end() || negd.find(ind-j) != negd.end()){
                continue;
            }

            cols.insert(j);posd.insert(j+ind);negd.insert(ind-j);
            board[ind][j]='Q';

            recur(ind+1,n,board,cols,posd,negd,res);

            cols.erase(j);posd.erase(j+ind);negd.erase(ind-j);
            board[ind][j]='.';

        }
        
    }
    vector<vector<string>> solveNQueens(int n) {
        if(n==1) return {{"Q"}};
        vector<vector<string>> res;
        vector<string> board(n,string(n,'.'));
        set<int> cols,posd,negd;
        
        recur(0,n,board,cols,posd,negd,res);
        return res;
    }
};
18465