Happy Number

Description

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, 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.

Return True if n is a happy number, and False if not.

Example:

Input: 19

Output: true

Explanation:

12 + 92 = 82

82 + 22 = 68

62 + 82 = 100

12 + 02 + 02 = 1

Code

Java

class Solution {
    public boolean isHappy(int n) {
        
        Set<Integer> set = new HashSet<>();
        
        while (n != 1) {
            if (set.contains(n)) return false;
            set.add(n);
            n = digitSquareSum(n);
        }
        return true;
    }
    public int digitSquareSum(int n) {
        int result = 0;
        while (n != 0) {
            int mod = n % 10;
            n /= 10;
            result += mod*mod;
        }
        return result;
    }
}

O(1) space

class Solution {
    public boolean isHappy(int n) {
        
        int slow = n;
        int fast = n;
        
        do {
            slow = digitSquareSum(slow);
            fast = digitSquareSum(fast);
            fast = digitSquareSum(fast);
        } while (slow != fast);
        return slow == 1;
        
    }
    public int digitSquareSum(int n) {
        int result = 0;
        while (n != 0) {
            int mod = n % 10;
            n /= 10;
            result += mod*mod;
        }
        return result;
    }
}