输入: 19
输出: true
解释: 
12 + 92 = 82
82 + 22 = 68
62 + 82 = 100
12 + 02 + 02 = 1

class Solution {
public:
    bool isHappy(int n) {
        if(n ==1 ) return true;
        if(n<4)
            return false;
        string s=to_string(n);
        unordered_set<int> before;
        while(true)
        {
        int res=0;
        for(auto c:s)
            res+=pow(c-'0', 2);
        s=to_string(res);
          //  cout<<res<<endl;
​
        if (res==1) return true;      
        if(before.count(res)) break;
        else        before.insert(res);
​
        }
        return false;
    }
};