@孩子，我无敌了，我才是C++真理

不懂不多，也就92行（不加多余换行，提交也正确）。
真理开始

#include <iostream>
#include <string>
#include <algorithm>
#include <cctype>
struct BigInt {
    std::string digits;
    BigInt() = default;
    explicit BigInt(const std::string& s) : digits(s) {}
    explicit BigInt(long long num) {
        if (num == 0) {
            digits = "0";
            return;
        }
        bool is_negative = num < 0;
        num = std::abs(num);
        while (num > 0) {
            digits.push_back(num % 10 + '0');
            num /= 10;
        }
        if (is_negative) {
            digits.push_back('-');
        }
        std::reverse(digits.begin(), digits.end());
    }
    BigInt operator+(const BigInt& other) const {
        BigInt result;
        int i = digits.size() - 1;
        int j = other.digits.size() - 1;
        int carry = 0;
        while (i >= 0 || j >= 0 || carry > 0) {
            int sum = carry;
            if (i >= 0 && digits[i] != '-') {
                sum += digits[i] - '0';
                i--;
            }
            if (j >= 0 && other.digits[j] != '-') {
                sum += other.digits[j] - '0';
                j--;
            }
            carry = sum / 10;
            result.digits.push_back(sum % 10 + '0');
        }
        std::reverse(result.digits.begin(), result.digits.end());
        return result;
    }
    friend std::ostream& operator<<(std::ostream& os, const BigInt& num) {
        os << num.digits;
        return os;
    }
    friend std::istream& operator>>(std::istream& is, BigInt& num) {
        is >> num.digits;
        if (!isValidInteger(num.digits)) {
            is.setstate(std::ios::failbit);
        }
        return is;
    }
private:
    static bool isValidInteger(const std::string& s) {
        if (s.empty()) return false;
        int start = 0;
        if (s[0] == '-') {
            if (s.size() == 1) return false;
            start = 1;
        }
        for (size_t i = start; i < s.size(); ++i) {
            if (!std::isdigit(s[i])) {
                return false;
            }
        }
        return true;
    }
};
void handleInput(BigInt& a, BigInt& b) {
    while (!(std::cin >> a)) {
        std::cin.clear();
        std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
    }
    while (!(std::cin >> b)) {
        std::cin.clear(); 
        std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n'); 
    }
}
void displayResult(const BigInt& a, const BigInt& b, const BigInt& sum) {
    std::cout << sum << std::endl;
}
int main() {
    BigInt a, b;
    handleInput(a, b);
    BigInt sum = a + b;
    displayResult(a, b, sum);
    return 0;
}