LeetCode Happy Number

class Solution {
public:
    bool isHappy(int n) {
        int last = n;
        
        unordered_set<int> exist;
        
        do {
            int sum = 0;
            exist.insert(last);
            while (last) {
                int digit = last % 10;
                sum += digit * digit;
                last /= 10;
            }
            if (sum == 1) {
                return true;
            }
            last = sum;
        } while (exist.count(last) < 1);
        return false;
    }
};

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