Best Time to Buy and Sell Stock

For one buy followed by one sell, scan once while maintaining minPrice, the lowest price among days strictly before or including the current scan position, and bestProfit, the best legal transaction found so far. On each price p, candidate profit is p minus minPrice; update bestProfit, then update minPrice if p is lower. The ordering ensures the buy day precedes the sell day. Initialize bestProfit to zero so decreasing prices correctly produce no transaction rather than a negative value. For [7, 1, 5, 3, 6, 4], minPrice becomes 1 and the best sale is 6 for profit 5. Test one day, monotonically decreasing prices, repeated values, and a gain on the final day. Do not sort prices because that destroys chronology, and do not use nested buy-sell loops. The scan is O(n) time and O(1) extra space. If multiple transactions are allowed, the invariant and solution change.