You're given daily stock prices. Find the maximum profit you can make by buying and selling the stock once. You must buy before you sell.

Implement a basic calculator to evaluate a simple expression string. The expression can contain non-negative integers, '+', '-', '*', '/' operators, and spaces; integer division should truncate toward zero.

You are given a string representing the order in which frogs are croaking. Determine the minimum number of frogs required to produce the entire croaking sequence, assuming they must croak in the order 'c', 'r', 'o', 'a', 'k'.