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

当某个数不是happy number的时候如何退出循环：

用set记录每个数字的 sum of every digit square，一旦当前数字的sum不能被加入set（说明之前出现过这个数，此后会一直进行相同的循环），返回false

class Solution {
    public boolean isHappy(int n) {
        Set<Integer> set = new HashSet<>();
        
        int sum = 0;
        while(sum != 1) {
            sum = 0;
            while(n != 0) {
                sum += (n % 10) * (n % 10);
                n /= 10;
            }
            if(!set.add(sum)) {
                return false;
            }
            set.add(sum);
            n = sum;
        }
        return true;
    }
}