202. Happy Number

 202. Happy Number    ( https://leetcode.com/problems/happy-number/ )

Write an algorithm to determine if a number n 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.

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.

Return true if n is a happy number, and false if not.

Example 1:

Input: n = 19

Output: true

Explanation:

1^2 + 9^2 = 82

8^2 + 2^2 = 68

6^2 + 8^2 = 100

1^2 + 0^2 + 0^2 = 1

Example 2:

Input: n = 2

Output: false

Constraints:

1 <= n <= 2^31 - 1


思路筆記：
首先要能求各位數的平方和
就是要 mod 10 平方
然後除 10，再mod 10 平方....
全部加起來

如果加起來的數字是 1 ，就 return true
要用 Set 記住加起來的數字 sum
如果 sum 曾經出現過，那就是繞回原點了
繼續往下走，也會再繞到這個數字，這樣就 return false

class Solution {

public:

    bool isHappy(int n) {

        set<int> numSet;

        int sum = 0;

        while (numSet.count(sum) == 0)

        {

            numSet.insert(sum);

            sum = 0;

            while (n != 0)

            {

                sum += pow(n % 10, 2) ;

                n /= 10;

            }

            n = sum;

            if (n == 1)

                return true;

        }

        return false;

    }

};

參考: 
https://www.youtube.com/watch?v=sYjmFRujbRI&list=PLY_qIufNHc29OLToHki4t0Z5KIhUzdYxD&index=3