big add/multi

string BigAdd(string s, string t){
        string res;
        int sz = max(s.size(), t.size());
        reverse(s.begin(), s.end());
        reverse(t.begin(), t.end());
        char carry = '0';
        int e = 0, f = 0;
        for(int i = 0; i < sz || carry != '0'; i++){
        e = (i >= s.size() ? 0 : s[i] - '0');
        f = (i >= t.size() ? 0 : t[i] - '0');
        int tmp = (carry - '0') + e + f;
        carry = (tmp/10) + '0';
        res.push_back(tmp%10 + '0');
        }
        reverse(res.begin(), res.end());
        while(res[0] == '0') res.erase(res.begin());
        return res;
    }

    string multiply(string s, string t) {
        if(s == "0" || t == "0") return "0";
        string res;
        reverse(s.begin(), s.end());
        reverse(t.begin(), t.end());
        for(int i = 0; i < t.size(); i++){
        string tmp = string(i, '0');
        char carry = '0';
        for(int j = 0; j < s.size(); j++){
            int m = (t[i]-'0') * (s[j]-'0') + (carry - '0');
            carry = (m/10) + '0';
            tmp.push_back(m%10 + '0');
        }
        tmp.push_back(carry);
        reverse(tmp.begin(), tmp.end());
        res = BigAdd(res, tmp);
        }
        while(res[0] == '0') res.erase(res.begin());
        return res;
    }