[LC 202] Happy number

Intuitive solution is easy: remember all the numbers that have been reached, as soon as find a number that has appeared before, we find a loop and return false.
Another interesting solution: cycle detection, have two pointers, slow and fast, and fast is going twice faster than the slow pointer. If they meet at one, return true, otherwise, return false.
Code template for cycle detection:
    int fast = start;
    int slow = start;
    do {
        slow = moveForward(slow) ;
        fast = moveForward(fast);
        fast  = moveForward(fast);
    } while(slow != fast);
    if (some condition) return someThing;
    else return otherThings;
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