202. 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: 
1^2 + 9^2 = 82
8^2 + 2^2 = 68
6^2 + 8^2 = 100
1^2 + 0^2 + 0^2 = 1

My Solutions:

用一个set记录所有出现过的数字（每一位数字平方的和）。如果一个数字曾经出现，说明陷入了一个循环，最后的结果无法变成1。一直检查直到这个数等于1。

lastDigit ^ 2 不成功 ， 因为^代表xor。如果要用math，用Math.pow(a,b) 代表a ^ b.

public boolean isHappy (int n) {
    Set<Integer> = new HashSet<>();
    while (n != 1) {
        int sum = 0;
        while (n > 0) {
            int lastDigit = n % 10;
            sum += lastDigit * lastDigit;
            n /= 10;
        }
        if (set.contains(sum)) return false;
        else set.add(sum);
        n = sum;
    }
    return true;
}