Problem

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
1^2 + 9^2 = 82
8^2 + 2^2 = 68
6^2 + 8^2 = 100
1^2 + 0^2 + 0^2 = 1

Solution

public class Solution {
      //param n an integer
      //return true if this is a happy number or false
    public boolean isHappy(int n) {
        Set<Integer> has = new HashSet<Integer>();
        while (n != 1) { //n等于1时跳出直接返回true；
            if (has.contains(n)) return false;
            has.add(n);
            int res = 0;
            //用HashSet存每一次变化后的n，若存入新值在set中存在，return false
            //从低位到高位 除以10取余求每一位的平方和
            while (n != 0) { //“位累加平方和循环”
                res += n % 10 * (n % 10);
                n /= 10;
            }
            n = res;//把累加后的值赋回n
        }
        return true;
    }
}