In the solution, it implemented merge sort to sort by the frequency first and then by value (in descending order). Is there any reason why one can't implement a custom compare function which does the same thing? In an interview setting, it saves a lot more time and is less error prone.

In C++, I implemented something like this:

bool compare(const std::pair<int, int> &a, const std::pair<int, int> &b) {
    if (a.first != b.first)
        return a.first < b.first;
    else
        return a.second > b.second;
}