HappyNumber.java

/*

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
 

Constraints:

1 <= n <= 231 - 1

*/



class HappyNumber {
    public int add(int a){
        int sum =0;
        while(a>0){
            int digit = a%10;
            sum += digit*digit;
            a = a/10;
        }
        return sum ;
    }
    public boolean isHappy(int n) {
        int slow = n ;
        int fast = n ;
        while(fast != 1){
            slow = add(slow);
            fast = add(add(fast));

            if(slow == fast && slow != 1){
                return false;
            }
            
        }
        return true;
    }
}