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

1^2 + 9^2 = 82

8^2 + 2^2 = 68

6^2 + 8^2 = 100

1^2 + 0^2 + 0^2 = 1

Solution:

  1. public class Solution {
  2.   public boolean isHappy(int n) {
  3.     Set<Integer> set = new HashSet<Integer>();
  5.     while (n > 1) {
  6.       int m = 0;
  8.       while (n > 0) {
  9.         int d = n % 10;
  10.         m += d * d;
  11.         n /= 10;
  12.       }
  14.       if (set.contains(m))
  15.         return false;
  17.       set.add(m);
  19.       n = m;
  20.     }
  22.     return true;
  23.   }
  24. }