Determine if a number is a 'happy number'. A happy number eventually reaches 1 when replaced by the sum of the squares of its digits, repeating the process until it does.

You are given an array where each element represents your maximum jump length at that position. Determine if you can reach the last index starting from the first index.

You are given a list of words. Find the longest possible chain of words where each word is formed by adding exactly one letter to its predecessor.