Happy Number

题目描述：

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

　　以前一直是按时间顺序做的，从这题开始按难易程度做。

我的解法：

bool isHappy(int n) {
    if(n <= 0)
        return false;
    set<int> st;
    while (1)
    {
        int tmp = 0;
        while (n > 0)
        {
            tmp += (n % 10)*(n % 10);
            n /= 10;
        }
        if(tmp == 1)
            return true;

        else
        {
            if(st.find(tmp) != st.end())
                return false;
            else
                st.insert(tmp);
        }
        n = tmp;
    }
}

简洁版：

bool isHappy(int n) {
    if(n <= 0)
        return false;
    unordered_set<int> st;
    while (n != 1 && st.find(n) != st.end())
    {
        st.insert(n);
        int tmp = 0;
        do 
        {
            tmp += (n % 10) * (n % 10);
        } while ( (n /= 10) > 0);
        n = tmp;
    }
    return n == 1;
}