backtracking n queen problem

class Solution {
public:
bool isSafe(int n,int r,int c,vector<int>upper_left_diag,vector<int>upper_right_diag,vector<int>upper_st){
if(upper_left_diag[n-1-r+c]==1){
    return false;
}
else if(upper_right_diag[r+c]==1){
    return false;
}
else if(upper_st[c]==1){
    return false;
}
return true;
}

void helper(int n,int row,vector<string>chess,vector<vector<string>>&ans,vector<int>upper_left_diag,vector<int>upper_right_diag,vector<int>upper_st){
if(row==n){
ans.push_back(chess);
return;
}
else{
    for(int i=0;i<n;i++){
bool is_safe=isSafe(n,row,i,upper_left_diag,upper_right_diag,upper_st);
if(is_safe){
chess[row][i]='Q';
upper_left_diag[n-1-row+i]=1;
upper_right_diag[row+i]=1;
upper_st[i]=1;
helper(n,row+1,chess,ans,upper_left_diag,upper_right_diag,upper_st);
chess[row][i]='.';
upper_left_diag[n-1-row+i]=0;
upper_right_diag[row+i]=0;
upper_st[i]=0;
}
    }
    return;
}

}
    vector<vector<string>> solveNQueens(int n) {
       vector<string>chess(n);
string s(n,'.');
for(int i=0;i<n;i++){
    chess[i]=s;
}
vector<vector<string>>ans;
vector<int>upper_left_diag(2*n,0);
vector<int>upper_right_diag(2*n,0);
vector<int>upper_st(n,0);
helper(n,0,chess,ans,upper_left_diag,upper_right_diag,upper_st);
return ans;
    }
};