https://leetcode.com/problems/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

如何判斷happy number邏輯題目已經交代的很清楚了，就是對數字的每一位digit的平方求和， 看這個和是否能為1. 注意有可能出現無限迴圈，所以用一個hashset來看當前平方的和有沒有出現過，如果有的話就會無限迴圈

public class Solution {
    public boolean isHappy(int n) {
        if(n <= 0){
            return false;
        }
        Set<Integer> set = new HashSet<>(); // to detect endless loop
        while(true){
            int sum = 0;//put it in the loop
            while(n > 0){
                int curDigit = n % 10;
                n /= 10;
                sum += curDigit * curDigit;
            }
            if(sum == 1)
                return true;
            else if(!set.add(sum)){
                return false;
            }else{
                n = sum;                
            }
        }
    }
}