Largest peak-to-trough drawdown

You are given a price path as a list. The maximum drawdown is the largest drop from a prior peak down to a later point: the biggest value of price[i] - price[j] where j comes after i. Report it as a nonnegative number, or 0 if prices never fall.

prices = [100, 90, 120, 80, 130]
-> 40          # peak 120 falls to 80

prices = [1, 2, 3, 4]
-> 0           # only rises; no drawdown

Return the maximum drawdown. Aim for $O(n)$ time and $O(1)$ space.