Challenge 26 - Rare Ingredients in Pizza Toppings

<p>Challenge 26: Rare Ingredients in Pizza Toppings</p><p><strong>Level:</strong> Easy</p><p><strong>Description:</strong> This week we propose a delicious (and, for some people, scary!) challenge: find the most uncommon pizza toppings. Given a dataset containing pizza options from several restaurants, along with their prices and ingredient lists, your task is to find the ingredients that are most rarely used -- i.e. the ones that appear the least in the ingredient lists. Go a step further if you will and extract the rarest ingredient pairs as well. <em>Buon appetito!</em></p>