202. Happy Number

Happy Number 的定義 Happy number - Wikipedia

• HashSet

class Solution {
public:
    bool isHappy(int n)
    {
        unordered_set<int> seen;
        while (n != 1 && !seen.count(n))
        {
            seen.insert(n);
            n = getNext(n);
        }
        return n == 1;
    }

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

• T: $O(\log n)$

• S: $O(\log n)$

• Floyd's Cycle-Finding Algorithm

class Solution {
public:
    bool isHappy(int n)
    {
        int slow = n;
        int fast = getNext(n);
        while (fast != 1 && slow != fast)
        {
            slow = getNext(slow);
            fast = getNext(getNext(fast));
        }
        return fast == 1;
    }

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

• T: $O(\log n)$
• S: $O(1)$