<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 isValid(vector<pair<int, int>> placedQueen, int m, int n)
    {
        for(auto [i, j]: placedQueen)
        {
            if(m == i || n == j || abs(m - i) == abs(n - j))
                return false;
        }
        return true;
    }
    void placeQueen(vector<vector<string>>& res, vector<string>& cur,
                    vector<pair<int, int>>& placedQueen, int i, int n)
    {
        if(i == n)
        {
            res.push_back(cur);
            return;
        }

        for(int j = 0; j < n; ++j)
        {
            if(isValid(placedQueen, i, j))
            {
                cur[i][j] = 'Q';
                placedQueen.push_back(pair<int, int>{i, j});
                placeQueen(res, cur, placedQueen, i + 1, n);
                placedQueen.pop_back();
                cur[i][j] = '.';
            }
        }
    }

    vector<vector<string>> solveNQueens(int n) {
        vector<string> cur(n, string(n, '.'));
        vector<pair<int, int>> placedQueen;
        vector<vector<string>> res;
        placeQueen(res, cur, placedQueen, 0, n);
        return res;
    }
};
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