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

• 12 + 92 = 82
• 82 + 22 = 68
• 62 + 82 = 100
• 12 + 02 + 02 = 1

分析：如果算得数字为1,则为Happy Number；如果进入无限循环，则不是Happy Number。

采用set进行存储，每进行一次平方求和（假设结果为s），对set内的数字进行判断：如果set中已经出现s，则表示会进入循环，返回false；否则，将s加入set。

运行时间：7ms

 1 class Solution {
 2 public:
 3     bool isHappy(int n){
 4         set<int> s;
 5         int recur = compute(n);
 6         while(true){
 7             if(recur == 1) return true;
 8             else{
 9                 if(s.find(recur) != s.end()) return false;
10                 else{
11                     s.insert(recur);
12                     recur = compute(recur);
13                 }
14             }
15         }
16     }
17     int compute(int n){
18         int sum = 0;
19         while(n){
20             int temp = n % 10;
21             sum += temp * temp;
22             n /= 10;
23         }
24         return sum;
25     }
26 };

Happy Number

原文：http://www.cnblogs.com/amazingzoe/p/4489798.html