202. Happy Number

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

解法要点：

难点在于，如何将结果永远不为1的数字停下来。后来参照大神，用set来检测是否开始循环运算。返回最后遇到重复数后，是否为1的布尔值。

class Solution(object):
    def isHappy(self, n):
        """
        :type n: int
        :rtype: bool
        """
        temp_set = set()

        while n not in temp_set:
            temp_set.add(n)
            n = sum([int(d)**2 for d in str(n)]) # 转换为list，并加到n里面

        return n == 1