Binary Search on the Answer — Koko Eating Bananas

Koko has piles of bananas and h hours before the guards return. Each hour
she picks one pile and eats speed bananas from it; if the pile has fewer, she
finishes it and waits for the next hour. Return the minimum integer speed
that lets her eat every banana within h hours.

There is no array to search here: search the answer space. Speeds go from
1 to max(piles), and "can she finish at speed s?" is monotonic: if speed
s works, every faster speed works too. Binary-search the smallest speed that
works, checking each candidate in O(n): O(n log max).

Examples

solve([3, 6, 7, 11], 8)        → 4
solve([30, 11, 23, 4, 20], 5)  → 30
solve([30, 11, 23, 4, 20], 6)  → 23

Constraints

• 1 <= piles.length <= h <= 10^9
• 1 <= piles[i] <= 10^9
• Hours for one pile at speed s = ceil(pile / s).