202. Happy Number

class Solution {
    
    private int notHappy(int n) {
        int nextnum = 0;
        while (n > 0) {
            int d = n % 10;
            n = n / 10;
            nextnum += d * d;
        }
        return nextnum;
    }
    public boolean isHappy(int n) {
        Set<Integer> hap = new HashSet<Integer>();
        
        while (n != 1 && !hap.contains(n)) {
            hap.add(n);
            n = notHappy(n);
        }
        
        return n == 1;
    }
}

Using recursion.

Floyd's Cycle-Finding Algorithm

public int getNext(int n) {
        int totalSum = 0;
        while (n > 0) {
            int d = n % 10;
            n = n / 10;
            totalSum += d * d;
        }
        return totalSum;
    }

    public boolean isHappy(int n) {
        int slowRunner = n;
        int fastRunner = getNext(n);
        while (fastRunner != 1 && slowRunner != fastRunner) {
            slowRunner = getNext(slowRunner);
            fastRunner = getNext(getNext(fastRunner));
        }
        return fastRunner == 1;
    }

사람들이 미쳤어요..^^