Best Time to Buy and Sell Stock

prices[d] is the price of one share on day d. You may buy one share on some day and sell it on a strictly later day, at most once. Write best_profit(prices: list[int]) -> int that returns the largest profit prices[sell] - prices[buy] you can make, or 0 if no trade makes money (you can always choose not to trade).

best_profit([8, 3, 6, 1, 5, 4])    # 4   (buy at 1, sell at 5)
best_profit([9, 7, 4, 2])          # 0   (prices only fall)
best_profit([2, 11])               # 9
best_profit([5])                   # 0

• 1 <= len(prices) <= 2 * 10^5; prices are in [0, 10^9].
• Trying every buy day with every later sell day is O(n²), too slow for the large tests. Aim for O(n) time and O(1) extra space.