<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 isSafe(int row, int col, unordered_map<int,int> rc, int n){
        int r = row, c = col;
        //towards top left
        while(r>=0 && c>=0){
            if(rc.find(r)!=rc.end() && rc[r]==c) return false;
            r--;
            c--;
        }
           
        r = row, c = col;
        //towards top right
        while(r>=0 && c<n){
            if(rc.find(r)!=rc.end() && rc[r]==c) return false;
            r--;
            c++;
        }
        
        return true;
    }
    
    void solve(int idx, vector<vector<string>> &res, int n, unordered_map<int,int> mp, unordered_map<int,int> rc, vector<string> asf){
        if(idx==n) {
            res.push_back(asf);
            return;
        }
        
        for(int j=0;j<n;j++){
            if(mp[j]!=1 && isSafe(idx,j,rc,n)){
                //queen placement
                mp[j] = 1;
                rc[idx] = j;
                asf[idx][j] = 'Q';
                
                solve(idx+1,res,n,mp,rc,asf);
                
                //queen replacement
                asf[idx][j] = '.';
                rc[idx] = -1;
                mp[j] = 0;
            }
        }
    }
    
    vector<vector<string>> solveNQueens(int n) {
        unordered_map<int,int> mp, rc;
        vector<vector<string>> res;
        string str(n,'.');
        vector<string> asf(n,str);
        
        solve(0,res,n,mp,rc,asf);
        
        return res;
    }
};
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