202. Happy Number

class Solution {
public:
    int next(int n) {
        int sum = 0;
        while (n) {
            sum += (n%10) * (n%10);
            n /= 10;
        }
        return sum;
    }
    
    bool isHappy(int n) {
        int fast = next(n), slow = n;
        
        while (fast != slow) {
            slow = next(slow);
            fast = next(next(fast));
        }
        
        return fast == 1;
    }
};

Space : O(1)