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);
}
}
};