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: 19 is a happy number

• 12 + 92 = 82
• 82 + 22 = 68
• 62 + 82 = 100
• 12 + 02 + 02 = 1

Solution: 线性的复杂度O(n)

public class Solution {
    public boolean isHappy(int n) {
        HashSet set = new HashSet<>();
        
        while (!set.contains(n)) {
            set.add(n);
            n = sum(getDigit(n));
            if (n == 1) {
                return true;
            }
        }
        return false;
    }
    
    public int sum(int[] arr) {
        int sum = 0;
        for(int i : arr) {
            sum += i * i;
        }
        return sum;
    }
    
    public int[] getDigit(int n) {
        String s = String.valueOf(n);
        
        int[] arr = new int[s.length()];
        int i = 0;
        while (n >= 1) {
            int m = n % 10;
            arr[i] = m;
            i++;
            n = n / 10;
        }
        return arr;
    }
}