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 `i`-th 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. ``` **Example 3:** ``` Input: prices = [1,2] Output: 1 ```