Solution: Sort Array by Increasing Frequency

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;
}