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: 19 is a happy number
- 12 + 92 = 82
- 82 + 22 = 68
- 62 + 82 = 100
- 12 + 02 + 02 = 1
思路分析：這題比較簡單，就是不斷的做各個位置數字的平方加和操作，看是否會產生1。但是有一個地方要注意，就是怎麼判斷出現了循環，這裏定義了一個Set，保存所有出現過的數字，對於計算得到的新的和，如果沒有在Set中出現，就添加到Set中繼續進行平方求和；否則，就是遇到了已經出現的數字，出現了循環，可以直接結束循環，返回false。
AC Code
public class Solution {
public boolean isHappy(int n) {
HashSet<Integer> appearedNums = new HashSet<Integer>();
while(!appearedNums.contains(n)){
appearedNums.add(n);
int sum = 0;
while(n != 0){
int digit = n % 10;
n /= 10;
sum += Math.pow(digit,2);
}
n = sum;
if(sum == 1) return true;
}
return false;
}
}