Given an array prices where prices[i] is a stock's price on day i, you may buy on one day and sell on a later day to maximize profit. A sale must happen strictly after the purchase. Return the maximum profit achievable, or 0 if no profitable trade exists. This looks like a search over every pair of days, but for any fixed sell day, only ONE earlier day could possibly be the best buy day: whichever one had the lowest price. Tracking that running minimum turns an O(n²) search into a single O(n) pass — and it's really the same Kadane's algorithmKadane's AlgorithmA single pass that, at each position, decides whether to extend the current running result or restart from here — used to find the best contiguous run in an array in O(n). idea in disguise: the day-to-day price differences form an array, and this problem's answer is exactly its maximum-sum contiguous subarray.