My code for solving Project Euler problem #55 is giving incorrect results. Specifically, my isLychrellNumber function gives way too many positive results, and also gives some false-negatives.

I’ve compared my code to other solutions but I can’t find the error.

My code:

// store single digits in an array, lowest digits come first
typedef std::vector<int> Digits;

// Adds two large numbers where y >= x
Digits add(Digits x, Digits y)
{
    Digits result{};
    int carry{ 0 };
    for (std::size_t i = 0; i < x.size(); ++i)
    {
        int digit{ x[i] + y[i] + carry };
        if (digit > 10)
        {
            carry = digit / 10;
            digit -= 10;
        }
        else
        {
            carry = 0;
        }
        result.push_back(digit);
    }
    if (y.size() > x.size())
    {
        for (std::size_t i = x.size(); i < y.size(); ++i)
        {
            int digit{ y[i] + carry };
            if (digit > 10)
            {
                carry = digit / 10;
                digit -= 10;
            }
            else
            {
                carry = 0;
            }
            result.push_back(digit);
        }
    }
    if (carry != 0)
        result.push_back(carry);

    return result;
}

template <typename T>
bool isPalindrome(const std::vector<T>& v)
{
    std::vector<T> reversed{ v };
    std::reverse(reversed.begin(), reversed.end());
    return v == reversed;
}

bool isLychrellNumber(int n)
{
    Digits numDigits{ numToDigits(n) };
    for (int i = 0; i < 60; ++i)
    {
        Digits flipped{ numDigits };
        std::reverse(flipped.begin(), flipped.end());
        
        auto sum{ add(numDigits, flipped) };
        
        if (isPalindrome(sum))
            return false;
        numDigits = std::move(sum);
    }
    return true;
}

int main()
{
    int answer{ 0 };
    std::cout << std::boolalpha << isLychrellNumber(349) << 'n';
    std::cout << std::boolalpha << isLychrellNumber(196) << 'n';
    for (int i = 0; i < 10000; ++i)
    {
        if (isLychrellNumber(i))
            ++answer;
    }

    std::cout << "Answer: " << answer;
    return 0;
}