In a group of people, find the "celebrity": someone who is known by everyone else, but doesn't know anyone themselves. You are given a way to query whether two people know each other.

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.

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.