LEETCODE 51. N-Queens

class Solution {
public:
bool issafe(int row,int col, vector<string>&board,int n)
{
    int dupcol=col;
    int duprow=row;
    //upper diagonal
    while(col>=0&&row>=0)
    {
        if(board[row][col]=='Q')
        return false;
        row--;
        col--;
    }
     row=duprow;
    col=dupcol;
    //left column 
    while(col>=0)
    {
        if(board[row][col]=='Q')
        return false;
        col--;
    }
       row=duprow;
    col=dupcol;
    //lower left diagonal
      while(row<n&&col>=0)
    {
        if(board[row][col]=='Q')
        return false;
        row++;
        col--;
    }
    return true;
}
void solve(int col,vector<vector<string>>&ans,vector<string>&board,int n)
{
    if(col==n)
    {
        ans.push_back(board);
        return ;
    }
    for(int row=0;row<n;row++)
    {
        if(issafe(row,col,board,n))
        {
            board[row][col]='Q';
            solve(col+1,ans,board,n);
            board[row][col]='.';
        }
    }
}
    vector<vector<string>> solveNQueens(int n) 
    {
        vector<vector<string>>ans;
        vector<string>board(n);
        string s(n,'.');
        for(int i=0;i<n;i++)
        board[i]=s;
        solve(0,ans,board,n);
        return ans;

        
    }
};