<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:
    bool isSafe(vector<vector<int>>&board,int row,int col,int n)
    {
        int x=row,y=col;
        while(x>=0 && y>=0)
        {
            if(board[x][y]==1) return false;
            x--;
            y--;
        }

        x=row,y=col;
        while(y>=0)
        {
            if(board[x][y]==1) return false;
            y--;
        }

        x=row,y=col;
        while(x<n && y>=0)
        {
            if(board[x][y]==1) return false;
            x++;
            y--;
        }
        return true;
    }
    void addSolution(vector<vector<int>>board,vector<vector<string>>&res, int n)
    {
        vector<string>ans;
        for(int i=0;i<n;i++)
        {
            string s;
            for(int j=0;j<n;j++)
            {
                if(board[i][j]==0) s+='.';
                else s+='Q';
            }
            ans.push_back(s);
        }
        res.push_back(ans);
    }
    void solve(vector<vector<int>>&board,vector<vector<string>>&res,int n,int col)
    {
        if(col==n)
        {
            addSolution(board,res,n);
            return ;
        }

        for(int row=0;row<n;row++)
        {
            if(isSafe(board,row,col,n))
            {
                board[row][col]=1;
                solve(board,res,n,col+1);
                board[row][col]=0;
            }
        }
    }
    vector<vector<string>> solveNQueens(int n) {
        vector<vector<string>>res;
        vector<vector<int>>board(n,vector<int>(n,0));
        solve(board,res,n,0);
        return res;
    }
};
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