N-Queens

// Leetcode --- NQueens

class Solution {
public:
    vector<vector<string>> res;
    bool Is_queen_safe(vector<string>& chess, int row , int col){
        
        for(int i = row , j= col ; i>=0 ;i--){// checking in upper direction
            if(chess[i][j] == 'Q'){
                return false;
            }   
        }
        
         for(int i = row , j= col ; i>=0 and j>=0;i-- ,j--){// checking in left diagonal
            if(chess[i][j] == 'Q'){
                return false;
            }   
         }
           
        for(int i = row , j= col ; i>=0 and j< chess.size();i-- , j++){// checking in right diagonal
            if(chess[i][j] == 'Q'){
                return false;
            }   
        }
        return true;
        
        
    }
    void helper(vector<string>& chess , int row ){
        if(row == chess.size()){
            res.push_back(chess);
            return ;
        }
        for(int i=0;i<chess.size() ;i++){

           if( Is_queen_safe(chess, row , i)){
                 chess[row][i] = 'Q';
                 helper(chess , row + 1);
                 chess[row][i] = '.';
            }
        }   
    }
    vector<vector<string>> solveNQueens(int n) {
    
        
        string s;
        for(int i=0;i<n;i++){
            s.push_back('.');
        }
        
       vector<string> chess(n, s);
        
      // initializing a chessboard with -
        // . . . .
        // . . . .
        // . . . .
        // . . . .
    // this is a chessboard containing string on each row initilizing with n dots 
        
        
       int row = 0;

       helper(chess ,  row );
        return res;
        
    }
};