XOR

struct Choice
{
    int a, b;
    int remove;
};

struct Ret
{
    long long result;
    vector<Choice> choices;
};

Ret bruteForceRecursion(const vector<int>& numbers, int currentResult, const vector<Choice>& choices)
{
    if (numbers.size() <= 1)
    {
        return Ret{currentResult, choices};
    }

    Ret max{0, vector<Choice>{}};
    for (int i = 0; i < numbers.size(); ++i)
    {
        for (int j = 0; j < numbers.size(); ++j)
        {
            if (j < i)
            {
                currentResult += numbers[i] ^ numbers[j];
                vector<int> numbers2_1(numbers), numbers2_2(numbers);
                vector<Choice> choices2_1(choices), choices2_2(choices);

                //prepare variables for recursion
                numbers2_1.erase(numbers2_1.begin() + i); //remove first
                numbers2_2.erase(numbers2_2.begin() + j); //remove second

                choices2_1.push_back(Choice{numbers[i], numbers[j], numbers[i]});
                choices2_2.push_back(Choice{numbers[i], numbers[j], numbers[j]});

                //recurse
                Ret r1 = bruteForceRecursion(numbers2_1, currentResult, choices2_1);
                Ret r2 = bruteForceRecursion(numbers2_2, currentResult, choices2_2);

                //choose higher result of current and two returned
                if (r1.result > max.result)
                {
                    max = r1;
                }
                if (r2.result > max.result)
                {
                    max = r2;
                }

                //restore environment for next loop iterations
                currentResult -= numbers[i] ^ numbers[j];
            }
        }
    }
    return max;
}