Leetcode #2 (

class Solution {
public:
    bool isHappy(int n) {
        set<int> st;
        int sum = n;
        while(true){
            sum = square_digit(sum);
            if(sum == 1)
                return true;
            else
            {
                if(st.find(sum) != st.end())
                    return false;
                else
                    st.insert(sum);
            }
        }
    }

    int square_digit(int n){
        int sum = 0;
        while(n){
            int digit = n % 10;
            sum += (digit*digit);
            n = n/10;
        }

        return sum;
    }
};