Умножение длинных

#include <fstream>
#include <algorithm>
#include <cstdlib>
#include <cstdio>
#include <string>
#include <vector>

typedef std::vector<int> lnum;
const int base = 1000 * 1000 * 1000;

void print(lnum a) {
    bool flag = false;
    FILE *fout = fopen("output.txt", "a");
//    fprintf(fout, "\n");
    for (int i = a.size() - 1; i >= 0; i--)
        if ((a[i] != 0) || (a[i] == 0 && flag == 1)) {
            if (flag == 0) {
                flag = true;
                fprintf(fout, "%d", a[i]);
            } else
                fprintf(fout, "%09d", a[i]);
        }

}

lnum toVec(std::string &str) {
    lnum toReturn;
    for (int i = (int) str.length(); i > 0; i -= 9)
        if (i < 9)
            toReturn.push_back(atoi(str.substr(0, i).c_str()));
        else
            toReturn.push_back(atoi(str.substr(i - 9, 9).c_str()));
    return toReturn;
}

void multiply(lnum a, lnum b) {
    lnum c(a.size() + b.size());
    for (size_t i = 0; i < a.size(); ++i)
        for (int j = 0, carry = 0; j < (int) b.size() || carry; ++j) {
            long long cur = c[i + j] + a[i] * 1ll * (j < (int) b.size() ? b[j] : 0) + carry;
            c[i + j] = int(cur % base);
            carry = int(cur / base);
        }
    while (c.size() > 1 && c.back() == 0)
        c.pop_back();
    print(c);
}

int main() {
    using namespace std;
    ifstream cin("input.txt");
    ofstream cout("output.txt");
    string first, second;
    lnum firstLnum, secondLnum;
    long long b;
    cin >> first >> second >> b;

    firstLnum = toVec(first);
    secondLnum = toVec(second);
    multiply(firstLnum, secondLnum);


    return 0;
}