<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 safePlace(vector<vector<char>> &board, int row, int col){
        int n = board.size();
        // -1, -1, -1, 1
        for(int i = 0; i<row; i++){
            if(board[i][col] == 'Q') return false;
        }

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

        x = row-1, y = col+1;
        while(x>=0 && y<n){
            if(board[x][y] == 'Q') return false;
            x = x - 1;
            y = y + 1;
        }
        return true;
    }

    void solve(vector<vector<char>> board, int ind, int n, vector<string> op, vector<vector<string>> &ans){
        cout << ind << endl;
        if(ind == n){
            ans.push_back(op);
            return;
        }

        for(int i=0; i<n; i++){
            string str;
            //cout << "checking at " << i << endl;
            if(safePlace(board, ind, i)){
                board[ind][i] = 'Q';
                for(int j=0; j<n; j++){
                    str.push_back('.');
                }
                str[i] = 'Q';

                //cout << "ind: " << ind << "   " << str << endl;

                op.push_back(str);
                solve(board, ind+1, n, op, ans);
                op.pop_back();
                board[ind][i] = '.';
            }
        }
    }

    vector<vector<string>> solveNQueens(int n) {
        vector<vector<string>> ans;
        vector<string> op;
        vector<vector<char>> board(n, vector<char>(n, '.'));
        solve(board, 0, n, op, ans);
        return ans;
    }
};
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