Koko has n piles of bananas. The guards are gone for h hours. Each hour, Koko picks a pile and eats up to k bananas from it. If the pile has fewer than k, she eats them all and stops eating until the next hour.

Return the minimum integer k such that she can eat all bananas within h hours.

Example: piles = [3,6,7,11], h = 8 → 4.

This problem uses binary search on the answer space — search for the smallest eating rate that is feasible.