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: 19 is a happy number

12 + 92 = 82
82 + 22 = 68
62 + 82 = 100
12 + 02 + 02 = 1

思路：
对这个数字的每个数位求平方和，如果如和为1或者平方和是之前出现过的就不进行求和，根据最后的结果判断是不是一个开心数字。

class Solution {    public boolean isHappy(int n) {        if (n < 1) {            return false;        }        // 用于保存中间出现的结果        HashSet<Integer> set = new HashSet<>(32);        int tmp;        int newN;        // n不为1，并且n的值不能重复出现，否则会死循环        while (n != 1 && !set.contains(n)) {            set.add(n);            newN = 0;            while (n > 0) {                tmp = n % 10;                n /= 10;                newN += tmp * tmp;            }            n = newN;        }        return n == 1;    }}