Happy number: detect cycles with a hash set

[vbrazo]

Write an algorithm to determine if a number n is happy.

A happy number is a number defined by the following process:

Starting with any positive integer, replace the number by the sum of the squares of its digits. Repeat the process until the number equals 1 (where it will stay), or it loops endlessly in a cycle which does not include 1. Those numbers for which this process ends in 1 are happy. Return true if n is a happy number, and false if not.

# Example 1:
#
# Input: n = 19
# Output: true
# Explanation:
# 12 + 92 = 82
# 82 + 22 = 68
# 62 + 82 = 100
# 12 + 02 + 02 = 1
#
# Example 2:
#
# Input: n = 2
# Output: false