202 Happy Number

来源：Leetcode

原帖：https://leetcode.com/problems/happy-number/

题目：
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

代码：

 class Solution {  
 public:  
   int nextNumber(int n) {  
     int res = 0;  
     while (n) {  
       int rem = n % 10;  
       res += rem * rem;  
       n /= 10;  
     }  
     return res;  
   }  
   bool isHappy(int n) {  
     if (n <= 0) return false;  
     unordered_set<int> set;  
     while (n != 1 && !set.count(n)) {  
       set.insert(n);  
       n = nextNumber(n);  
     }  
     return n == 1;  
   }  
 };