## Happy Number

### 描述

Write a function to determine if a number is "happy number".

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

$1^2 + 9^2 = 82$

$8^2 + 2^2 = 68$

$6^2 + 8^2 = 100$

$1^2 + 0^2 + 0^2 = 1$

### 代码

// Happy Number
// Time complexity: ?, Space complexity: ?
public class Solution {
public boolean isHappy(int n) {
final Set<Integer> existed = new HashSet<>();
while (true) {
int sum = 0;
while (n > 0) {
int digit = n % 10;
sum += digit * digit;
n /= 10;
}
if (existed.contains(sum)) {
return sum == 1;
} else {
existed.add(sum);
n = sum;
}
}
}
}