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
Credits:
Special thanks to @mithmatt and @ts for adding this problem and creating all test cases.

使用set即可，注意這道題最終輸出要麼是因為1結束，要麼是由於出現環路結束。注意set返回的是pair，所以要判斷set的第二個元素。

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