<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<bool>> block(vector<vector<bool>> done, int r, int c, int n) {
        // Block all vertical places
        for (int i = 0; i < n; i++)
            done[i][c] = true;

        // Block all horizontal places
        for (int j = 0; j < n; j++)
            done[r][j] = true;

        // Block right hand diagonal places
        int i = r + 1;
        int j = c + 1;
        while (i < n && j < n) {
            done[i][j] = true;
            i++;
            j++;
        }

        // Block left hand diagonal places
        i = r + 1;
        j = c - 1;
        while (i < n && j >= 0) {
            done[i][j] = true;
            i++;
            j--;
        }

        return done;
    }
    string pattern(int c, int n) {
        string s(n, '.');
        s[c] = 'Q';
        return s;
    }

    void helper(int n, int r, vector<vector<bool>> done, vector<string> temp,
                vector<vector<string>>& ans) {
        // Part of answer
        if (r == n) {
            ans.push_back(temp);
            return;
        }

        for (int i = 0; i < n; i++) {
            if (done[r][i] == false) {
                vector<vector<bool>> next_done = block(done, r, i, n); 
                string s = pattern(i, n);
                temp.push_back(s);
                helper(n, r + 1, next_done, temp, ans);
                temp.pop_back();
            }
            else
                continue;
        }
        return;
    }

    vector<vector<string>> solveNQueens(int n) {
        vector<vector<string>> ans;
        vector<vector<bool>> done(n, vector<bool>(n, false));
        helper(n, 0, done, {}, ans);
        return ans;
    }
};
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