Max profit from a single trade

You're given the intraday price path of a stock as a list. You may buy once and sell once, and the sell must come after the buy. Short selling is not allowed; if no profitable trade exists, do nothing.

prices = [7, 1, 5, 3, 6, 4]
-> 5            # buy at 1, sell at 6

prices = [7, 6, 4, 3, 1]
-> 0            # prices only fall; skip the trade

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