leetcode N-Queens/N-Queens II, backtracking, C++

thanks to the video of stanford cs106b lecture 10 by Julie Zelenski for the nice explanation of recursion and backtracking, highly recommended.

//N-Queens(12ms)

class Solution {
    vector<vector<string>> ans;
    int N;
    //int numofsol;

    void AddTo_ans(string &conf) {
        vector<string> vecstrtmp;
        string strtmp(N,‘.‘);
        for(auto ind: conf) {
            vecstrtmp.push_back(strtmp);
            vecstrtmp.back()[(size_t)ind-1]=‘Q‘;
        }
        ans.push_back(vector<string>());
        ans.back().swap(vecstrtmp);
    }
    void RecListNQueens(string soFar, string rest) {
        if(rest.empty()) {
            //++numofsol;
            AddTo_ans(soFar);
        }
        else {
            int i,j,len=soFar.size(), flag;
            for(i=0;i<rest.size();++i) {
                for(j=0;j<len;++j) {
                    flag=1;
                    if(soFar[j]-rest[i]==len-j || soFar[j]-rest[i]==j-len) {
                        flag=0; break;
                    }
                }
                if(flag) RecListNQueens(soFar+rest[i],rest.substr(0,i)+rest.substr(i+1));
            }
        }
    }
public:
    vector<vector<string>> solveNQueens(int n) {
        ans.clear();
        //numofsol=0;
        N=n;
        string str;
        for(int i=1;i<=n;++i) str.push_back(i);
        RecListNQueens("", str);
        return ans;
    }
};

//with a tiny modification,
//N-Queens II (8ms)

class Solution {
    //vector<vector<string>> ans;
    int N;
    int numofsol;

    /*void AddTo_ans(string &conf) {
        vector<string> vecstrtmp;
        string strtmp(N,‘.‘);
        for(auto ind: conf) {
            vecstrtmp.push_back(strtmp);
            vecstrtmp.back()[(size_t)ind-1]=‘Q‘;
        }
        ans.push_back(vector<string>());
        ans.back().swap(vecstrtmp);
    }*/
    void RecListNQueens(string soFar, string rest) {
        if(rest.empty()) {
            ++numofsol;
            //AddTo_ans(soFar);
        }
        else {
            int i,j,len=soFar.size(), flag;
            for(i=0;i<rest.size();++i) {
                for(j=0;j<len;++j) {
                    flag=1;
                    if(soFar[j]-rest[i]==len-j || soFar[j]-rest[i]==j-len) {
                        flag=0; break;
                    }
                }
                if(flag) RecListNQueens(soFar+rest[i],rest.substr(0,i)+rest.substr(i+1));
            }
        }
    }
public:
    int totalNQueens(int n) {
        //ans.clear();
        numofsol=0;
        N=n;
        string str;
        for(int i=1;i<=n;++i) str.push_back(i);
        RecListNQueens("", str);
        return numofsol;
    }
};