LeetCode 202. Happy Number

class Solution {
public:
    int sum (int n){  //caculate 
        int num[] ={0,1,4,9,16,25,36,49,64,81};
        int res = 0;
        while(n !=0){
            res += num[n%10];
            n /= 10;
        }
        return res;
    }
    
    bool isHappy(int n) {

        set<int> fre;
        int res = n;
        while(true ){
            if (res == 1)
                return true;
            if(fre.count(res) == 1)
                return false;
            fre.insert(res);
            res = sum(res);    
            
        }
        
    }
        
};