BigInt

main.c

#include <iostream>
#include <vector>
#include <iomanip>

#define lenBigInt 9
#define divBigInt 1000000000

// Возвращет вектор в обратном порядке, 12 345 678 987 654 123 -> 987654123 12345678
std::vector<unsigned int> parseBigIntReverced(std::string str) {
    int i = str.size();
    std::vector<unsigned int> vec;

    while (i >= 0) {
        int offset = std::min(i, lenBigInt);
        vec.push_back(std::atoi(str.substr(i - offset, offset).c_str()));
        i -= lenBigInt;
    }
    return vec;
}

std::vector<unsigned int> sumBigInt(std::vector<unsigned int> numbers1, std::vector<unsigned int> numbers2) {
    size_t maxOfNumbers = std::max(numbers1.size(), numbers2.size());
    std::vector<unsigned int> vec(maxOfNumbers + 1, 0);
    for (int i = 0; i < maxOfNumbers; i++) {
        unsigned long x, y;
        if (i > numbers1.size() - 1) {
            x = 0;
        } else {
            x = numbers1[i];
        }

        if (i > numbers2.size() - 1) {
            y = 0;
        } else {
            y = numbers2[i];
        }
        unsigned long sum = x + y;
        vec[i] += sum % divBigInt;
        vec[i + 1] += sum / divBigInt;
    }
    return vec;
}

void printBigInt(std::vector<unsigned int> vec) {
    int i = vec.size() - 1;
    while (i >= 0) {
        if (vec[i] != 0) {
            break;
        }
        i--;
    }
    if (i >= 0) {
        std::cout << vec[i];
        i--;
        while (i >= 0) {
            std::cout << std::setfill('0') << std::setw(lenBigInt) << vec[i];
            i--;
        }
    } else {
        std::cout << 0;
    }
    std::cout << std::endl;
}

int main(int argc, char **argv) {
    std::string str;
    std::cout << "Input first number:" << std::endl;
    std::cin >> str;
    std::vector<unsigned int> numbers1 = parseBigIntReverced(str);
    std::cout << "Input second number:" << std::endl;
    std::cin >> str;
    std::vector<unsigned int> numbers2 = parseBigIntReverced(str);
    std::vector<unsigned int> sum = sumBigInt(numbers1, numbers2);
    std::cout << "Result a + b =:" << std::endl;
    printBigInt(sum);
    return 0;
}