LeetCode 410: Split Array Largest Sum
Function signatures
You've made the best out of a slightly silly situation. I think your function signatures, while abiding to the template, have improved since the last post of yours that I reviewed.
I'm not sure whether this will break LeetCode compatibility, but if you want to enforce that no one can instantiate this class, try making a private default constructor.
Mean
A simpler way to express the arithmetic mean than
        size_t mid = lo + (hi - lo) / 2;

is
        size_t mid = (hi + lo) / 2;