202 Happy Number.java

/* Leetcode 202. Happy Number
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
*/
class Solution {
    public boolean isHappy(int n) {
        Set<Integer> set = new HashSet<>();
        while(n != 1 && !set.contains(n)){ // Check whether it gets stuck in a cycle.
            set.add(n);
            n = nextNumber(n);
        }
        
        return n == 1; // Only when the result equals 1 should be true(happy number)
    }
    
    public int nextNumber(int n){
        int sum = 0;
        while(n > 0){
            int temp = n % 10; // Use '%' can get the last right side digit in n
            n = n / 10; // Use '/' can get the digits start at tens place
            sum += temp * temp; // Get the sum of the squares of its digits.
        }
        
        return sum; // Get the sum of the squares of its digits.
    }
}