No matter what language you use on the OJ, you may refer to for a solution. The C++ is rewritten below, just really concise.

1 class Solution {2 public:3     bool isUgly(int num) {4         for (int i = 2; i < 6 && num; i++)5             while (!(num % i)) num /= i;6         return num == 1;7     }8 };

Note that i = 4 will simply be skipped after we try to divide by 2 as much as possible.