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: 

Input: 19
Output: true
Explanation: 
12 + 92 = 82
82 + 22 = 68
62 + 82 = 100
12 + 02 + 02 = 1

題解：如果數字的和開始出現重複且沒到 $1$ 那麼就是 $false$

程式碼：

class Solution {
public:
    bool isHappy(int n) {
        set<int> s;
        while(n != 1) {
            int sum = 0;
            while(n) {
                sum += (n % 10) * (n % 10);
                n /= 10;
            }
            n = sum;
            if(s.count(n)) break;
            else s.insert(n);
        }
        if(n == 1) return true;
        else return false;
    }
};