Problem - Best Time to Buy and Sell Stock

Given an array where $prices[i]$ is the stock price on day $i$, find the maximum profit from one buy and one sell.

You must buy before you sell.

For example, $prices = [7,1,5,3,6,4]$ gives profit $5$ (buy at $1$, sell at $6$).

This looks like a search problem, but you can solve it greedily.

Think about what you need to track as you scan through prices. Constraint: $n \le 10^5$. Checking all $O(n^2)$ time using $O(1)$ space buy-sell pairs times out.