Write an algorithm to determine if a number 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, and 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 numbers.

Example:

Input: 19

Output: true

Explanation:

1² + 9² = 82

8² + 2² = 68

6² + 8² = 100

1² + 0² + 0² = 1

这道题提示会有infinate loop，这就说明是个cycle，利用这一点，需要建立一个set，把中间的每个得到的number都加到这个set里面，如果已经有了，就说明这是个cycle，直接return false，不然会得到1。只要有cycle就要想到set或者cycle list。

注意一个小细节 divmod的应用

class Solution:
    def isHappy(self, n: int) -> bool:
        seen = set()
        while n != 1 and n not in seen:
            seen.add(n)
            n = self.caculate_next(n)
        return True if n == 1 else False
            
    def caculate_next(self, n: int) -> int:
        total_sum, num = 0, n
        while num > 0:
            num, digit = divmod(num, 10)
            total_sum += digit**2
        return total_sum
        

[LeetCode] 202. Happy Number