【题目描述】
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.
写一个算法来判断一个数是不是"快乐数"。
一个数是不是快乐是这么定义的：对于一个正整数，每一次将该数替换为他每个位置上的数字的平方和，然后重复这个过程直到这个数变为1，或是无限循环但始终变不到1。如果可以变为1，那么这个数就是快乐数。
【题目链接】
www.lintcode.com/en/problem/happy-number/
【题目解析】
判断一个数字是否为Happy Number。这道题跟求无限循环小数很像，最好维护一个HashSet，假如遇见重复，则返回false。否则替换n为digits square root sum，当n == 1时循环结束返回true。
【参考答案】
