LeetCode 263. Ugly Number 丑数（Java）

题目：

Write a program to check whether a given number is an ugly number.

Ugly numbers are positive numbers whose prime factors only include 2, 3, 5.

Example 1:
Input: 6
Output: true
Explanation: 6 = 2 × 3

Example 2:
Input: 8
Output: true
Explanation: 8 = 2 × 2 × 2

Example 3:
Input: 14
Output: false
Explanation: 14 is not ugly since it includes another prime factor 7.

Note:

• 1 is typically treated as an ugly number.
• Input is within the 32-bit signed integer range: [−231, 231 − 1].

解答：

采用递归思路即可。如果一个数 num 为丑数，则 num 不断递归整除2,3,5直到最后，结果总是为1。

class Solution {
    public boolean isUgly(int num) {
        if(num<=0) {
            return false;
        }
        if(num==1){
            return true;
        }
        if(num%2==0) {
            num=num/2;
            return isUgly(num);
        }
        if(num%3==0) {
            num=num/3;
            return isUgly(num);
        }
        if(num%5==0) {
            num=num/5;
            return isUgly(num);
        }
        return false;
    }
}