<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 {
    bool possible(vector<string> cur, int i, int n){
        int straight = i;
        int left = i-1;
        int right = i+1;
        for(int j=cur.size()-1; j>=0; j--){
            if(cur[j][straight] == 'Q') return 0;
            if(left>=0){
                if(cur[j][left] == 'Q') return 0;
                left--;
            }
            if(right<n){
                if(cur[j][right] == 'Q') return 0;
                right++;
            }
        }
        return 1;
    }
    void rec(int n, int j, vector<string> cur, vector<vector<string>>& ans){
        if(j==n) {
            ans.push_back(cur);
            return;
        }
        string curs;
        for(int i=0; i<n; i++) curs.push_back('.');
        for(int i=0; i<n; i++){
            if(!possible(cur, i, n)) continue;
            curs[i] = 'Q';
            cur.push_back(curs);
            rec(n, j+1, cur, ans);
            cur.pop_back();
            curs[i] = '.';
        }
    }
public:
    vector<vector<string>> solveNQueens(int n) {
        vector<vector<string>> ans;
        vector<string> cur;
        rec(n, 0, cur, ans);
        return ans;
    }
};
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