Happy Number | Leetcode

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

Credits:
Special thanks to @mithmatt and @ts for adding this problem and creating all test cases.

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Solution: Maintain a hash set to remember all the numbers that we have already seen. Loop infinitely. If we get 1, then n is a happy number. If we get a number that we have already seen, that means we are going to loop infinitely, so n is not a happy number.

public class Solution {
    public boolean isHappy(int n) {
        Set<Integer> seen = new HashSet<>();
        seen.add(n);
        
        while (true) {
            if (n == 1) return true;
            
            String s = Integer.toString(n);
            n = 0;
            for (int i = 0; i < s.length(); i++) {
                int c = s.charAt(i) - '0';
                n += c * c;
            }
            
            if (seen.contains(n)) return false;
            seen.add(n);
        }
    }
}