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.

思路：
按照題目要求寫一個函數不斷求出一個數字的next，判斷next是否等于1，不等于1就循環或遞歸繼續求下去。
坑：如果這個數字不是happy num，那么它的next會是一個死循環，因此需要用一個hashset來記錄已經生成過的數字。

public boolean isHappy(int n) {
    HashSet<Integer> set = new HashSet<>();
    while (set.add(n)) {
        int res = 0, tmp = 0;
        while (n > 0) {
            tmp = n % 10;
            res += tmp * tmp;
            n /= 10;
        }
        if (res == 1) {
            return true;
        } else {
            n = res;
        }
    }

    return false;
}