Compute maximum run-up in one pass

Maximum run-up is the largest percentage gain from a running trough to a subsequent peak of an equity curve, the upside twin of maximum drawdown:

$\text{MRU} = \max_{i \le j} \frac{E_j - E_i}{E_i}$

where $E_i$ is the equity at time $i$ (assume all values positive).

equity = [100, 80, 130, 90, 150]
-> 0.875         # trough 80 up to peak 150

Return the maximum run-up as a fraction, in $O(n)$ time and $O(1)$ space. The naive all-pairs scan is $O(n^2)$.