<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>> solveNQueens(int n) {
        vector <vector <string>> ans;

        string s = "";

        for (int i = 0;i < n;i++) s += '.';

        vector <string> board(n, s);

        vector <vector <int>> myQ;

        dfs(0, board, myQ, ans);

        return ans;
    }

    void dfs(int row, vector <string> board, vector <vector <int>> myQ, vector <vector <string>> &ans) {
        int n = board.size();

        for (int column = 0;column < n;column++) {
            bool flag(true);
            for (auto xy : myQ) {
                if (xy[1] == column or abs(column - xy[1]) == abs(row - xy[0])) {
                flag = false;
                break;
                }
            }
            if (flag) {
                board[row][column] = 'Q';
                myQ.push_back({row, column});
                if (row == n - 1) ans.push_back(board);
                else dfs(row + 1, board, myQ, ans);
                board[row][column] = '.';
                myQ.erase(myQ.end() - 1);
            }
        }
    }
};
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