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: 

Input: 19
Output: true
Explanation: 
12 + 92 = 82
82 + 22 = 68
62 + 82 = 100
12 + 02 + 02 = 1

Solution – Efficient approach

public class Solution {
    public bool IsHappy(int n) {
        int slow = n, fast = n;
        
        do
        {
            slow = FindSquareSum(slow);    
            
            fast = FindSquareSum(FindSquareSum(fast));
        }while(slow != fast);
        
        return slow == 1;
    }
    
    public int FindSquareSum(int n)
    {
        int squareSum = 0;
        
        while(n != 0)
        {
            squareSum += (n%10) * (n%10);
            n = n/10;
        }
        
        return squareSum;
    }
}

Time Complexity : O(n)

Space Complexity : O(1)