Additive Number

class Solution {
public:
    bool additive(string num, long frst, long sec)
    {
        if(num.empty()) return 1;
        long sum = frst + sec;
        string summ = to_string(sum);
        string prefix = num.substr(0, summ.size());
        if(prefix != summ) return 0;
        return additive(num.substr(summ.size()), sec, sum);
    }
    
    bool isAdditiveNumber(string num) {
        int len = num.size();
        if(len < 3) return 0;
        for(int i = 1; i < len-1; i++)
        {   if(i>1 and num[0]=='0') break;
            long frst = atoi(num.substr(0,i).c_str());
            for(int j = i+1; j < len; j++)
            {
                if(j-i > 1 and num[i]=='0') break;
                long sec = atoi(num.substr(i, j-i).c_str());
                if(additive(num.substr(j), frst, sec)) return 1;
            }
        }
        return 0;
    }
};