LeetCode 202. Happy Number

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

O(n) memory complexity solution has nothing special. Just read the problem carefully.....

class Solution {
private:
     set<int> integers;
public:
   static int sumOfDigit(int n) {
        int sum = 0;
        while(n) {
            int tmp = n % 10;
            sum += tmp * tmp;
            n = n / 10;
        }
        return sum;
    }
    bool isHappy(int n) {
        int sum = sumOfDigit(n);
        if(sum == 1) return true;
        else {
            if(integers.find(sum) != integers.end()) return false;
            integers.insert(sum);
            return isHappy(sum);
        }
    }
};