263.Ugly Number

给定一个数，判断其质因数是否是2，3，5。
Input: 6
Output: true
Explanation: 6 = 2 × 3

Input: 14
Output: false
Explanation: 14 is not ugly since it includes another prime factor 7.

思路：对2，3，5取余判断，可以先分别对2，3，5取余，再判断，减少重复判断次数。

bool isUgly(int num) {
    if (num <= 0) return false;
    if (num == 1) return true;
    while (num > 1) {
        if (num % 2 == 0) num /= 2;
        if (num % 3 == 0) num /= 3;
        if (num % 5 == 0) num /= 5;
        if (num != 1 && num % 2 != 0 && num % 3 != 0 && num % 5 != 0) return false;
    }
    return true;
}