题目
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, 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.
Return True if n is a happy number, and False if not.
解题思路
按照题目要求求得每一步结果，把结果存到一个set中，如果出现重复，就证明有环，就不会到达1，返回False，遇到1就返回True
可以使用类似快慢指针的方式来找环，同时进行运算，一个每次求下一数，另一个每次求再下一个数，如果有环，这两个数就会相同，就不用额外的空间存储（感谢b站观众 92号汽油_dbs 提供）
代码
class Solution:
def isHappy(self, n: int) -> bool:
s = {n}
while n != 1:
tmp = sum([int(c) ** 2 for c in str(n)])
if tmp in s:
return False
s.add(tmp)
n = tmp
return True
