[LeetCode] Happy Number

Well, no matter whether the number is happy or not, its sum-of-squared-digits sequance has a cycle. Well, do you still remember the algorithm for detecting cycles in linked lists? Yeah, use a fast and a slow pointer. That's also applicable to this problem.

The code is as follows (idea from here).

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