// Best Time to Buy and Sell Stock II — EASY
// Category: greedy
You are given an integer array `prices` where `prices[i]` is the price of a given stock on the `i`-th day.
On each day, you may decide to buy and/or sell the stock. You can only hold **at most one share** of the stock at any time. However, you can buy it then immediately sell it on the **same day**.
Find and return the **maximum profit** you can achieve.
Example: prices = [7,1,5,3,6,4]
Output: 7