LeetCode202 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: 

Input: 19
Output: true
Explanation: 
12 + 92 = 82
82 + 22 = 68
62 + 82 = 100
12 + 02 + 02 = 1

题源：here

思路：

我们使用一个set来保存所有出现过的数，如果这个数再一次出现，那么说明开始循环了；如果我们发现这个数最终累加到1，那么直接返回true。

class Solution {
public:
    bool isHappy(int n) {
        set<int> s;
        s.insert(n);
        while(n!=1){
            n = helper(n);
            if(s.find(n) == s.end()) s.insert(n);
            else return false;
        }
        return true;
    }
    int helper(int a){
        vector<int> ivec;
        while(a){
            int tmp = a % 10;
            ivec.push_back(tmp*tmp);
            a /= 10;
        }
        return accumulate(ivec.begin(), ivec.end(), 0);
    }
};