Leetcode 202 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
判断一个数是否为快乐数。

正常做法是不断迭代，用map记录出现过的数字，如果出现了则表示循环返回false，变成1则返回true。

做完正常的做法去看了一下discuss，有人通过观察发现，非快乐数都会在循环中出现4，

维基百科，涨姿势了，https://en.wikipedia.org/wiki/Happy_number#Sequence_behavior

class Solution {
public:
int fun(int x)
{
int ret = 0;
while(x)
{
int now = x %10;
x /= 10;
ret += now * now;
}
return ret;
}
bool isHappy(int n) {
while(1)
{
if(n == 1) return 1;
if(n == 4) return 0;
n = fun(n);
}
}
};