LeetCode Happy Number

 1 class Solution {
 2 public:
 3     bool isHappy(int n) {
 4         int last = n;
 5         
 6         unordered_set<int> exist;
 7         
 8         do {
 9             int sum = 0;
10             exist.insert(last);
11             while (last) {
12                 int digit = last % 10;
13                 sum += digit * digit;
14                 last /= 10;
15             }
16             if (sum == 1) {
17                 return true;
18             }
19             last = sum;
20         } while (exist.count(last) < 1);
21         return false;
22     }
23 };

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