Determine if a string containing parentheses is valid. A valid string has matching and properly nested opening and closing parentheses.

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.

Determine if a given string is a palindrome, considering only alphanumeric characters and ignoring cases. Essentially, check if the string reads the same forwards and backward after removing non-alphanumeric characters and converting to lowercase.