// Best Time to Buy and Sell Stock — EASY
// Category: array
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: prices = [7, 1, 5, 3, 6, 4]
Output: 5