Write an algorithm to determine if a number n 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. 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. Return true if n is a happy number, and false if not.

Example 1:

Input: n = 19
Output: true
Explanation:
12 + 92 = 82
82 + 22 = 68
62 + 82 = 100
12 + 02 + 02 = 1

Example 2:

Input: n = 2
Output: false

<解題>

1. 先建立一個HashSet，來檢測是否有出現過的數字
2. 如果不為1(1是快樂數)且還沒有重複時，就進入迴圈，同時加到seen裏面，以便之後繼續檢測
3. 由於每次需要計算各位數字的平方和，故進入while迴圈裡處理：先取出最後一位數字，再平方，後來再把最後一位數字去掉…
4. 更新n=sum，進入下一個迴圈，直到重複循環或找到n==1（快樂數）為止

class Solution {
    public boolean isHappy(int n) {
        Set<Integer> seen = new HashSet<>(); //存儲已經出現過的數字，以檢測是否陷入了循環

        while (n != 1 && !seen.contains(n)) {
            seen.add(n);
            
            int sum = 0;
            while (n != 0) { // n=0的話，代表每一位數都處理完了
                int digit = n % 10; //最後一位數字取出
                sum += digit * digit; //最後一位數字取出後平方
                n /= 10; //將 n 除以 10 得到去掉最後一位數字後的結果
            }
            n = sum;
        }
        
        // If n is 1, it is a happy number; otherwise, it is not
        return n == 1;
    }
}

Time: O() Space: O()