Best Time to Buy and Sell Stock

You are given an array `prices` where `prices[i]` is the price of a given stock on the `ith` day. You want to maximize your profit by choosing a **single day** to buy one stock and choosing a **different day in the future** to sell that stock. Return _the maximum profit you can achieve from this transaction_. If you cannot achieve any profit, return `0`.   **Example 1:** **Input:** prices = [7,1,5,3,6,4] **Output:** 5 **Explanation:** Buy on day 2 (price = 1) and sell on day 5 (price = 6), profit = 6-1 = 5. Note that buying on day 2 and selling on day 1 is not allowed because you must buy before you sell. ``` **Example 2:** **Input:** prices = [7,6,4,3,1] **Output:** 0 **Explanation:** In this case, no transactions are done and the max profit = 0. ```   **Constraints:** - `1 <= prices.length <= 105` - `0 <= prices[i] <= 104`