ZSBK

#include <iostream>
#include <string>

using namespace std;

class Data{
    public:
    string num;
    Data(int n){
        num = to_string(n);
    }
    static Data minus(Data a, Data b){
        int n = b.num.length()-1;
        int m = a.num.length()-1;
        int s = 0, c = 0;
        int h = 0;
        for(int i=n, j=m;i>=0;i--,j--){
            int op1 = a.num[j]-'0'-c;
            int op2 = b.num[i]-'0';
            //cout<<"op:"<<op1<<" "<<op2<<endl;
            if(op1<op2){
                c=1;
            }else{
                c=0;
            }
            int dif = op1+c*10-op2;
            s = dif % 10;
            a.num[j] = s+'0';
            h = j;
        }
        // Deal with the head
        for(int i = h-1;c!=0&&i>=0;i--){
            int op1 = a.num[i]-'0';
            int op2 = c;
            if(op1<op2){
                c=1;
            }else{
                c=0;
            }
            int dif = op1+c*10-op2;
            s = dif% 10;
            a.num[i] = s+'0';
        }
        return a;
    }
    static Data mul(Data a, int b){
        int n = a.num.length()-1;
        int s = 0, c = 0;
        for(int i=n;i>=0;i--){
            int op1 = a.num[i] - '0';
            int prd = op1 * b + c;
            c = prd/10;
            s = prd%10;
            a.num[i] = s + '0';

        }
        if(c!=0){
            a.num = to_string(c)+a.num;
        }
        return a;

    }


};

Data left_cows(int n ){
  if(n<=1){
      Data d1(10);
      return d1;
  }else{
      Data t(n-1);
      Data dn_1 = Data::mul(left_cows(n-1),3);
      Data n_1 = Data::mul(t,2);
      Data ret = Data::minus(dn_1,n_1);
      cout<<"n:"<<n<<endl;
      cout<<"dn_1: "<<dn_1.num<<" "<<"n_1: "<<n_1.num<<endl;
      cout<<"dn: "<<ret.num<<endl;
      return ret;
  }   


};
int main() {
    Data ret = left_cows(100);
    cout<<ret.num<<endl;
    std::cout << "Hello World!\n";
}