263. Ugly Number

problem

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. For example, 6, 8 are ugly while 14 is not ugly since it includes another prime factor 7.

Note that 1 is typically treated as an ugly number.

Credits:
Special thanks to @jianchao.li.fighter for adding this problem and creating all test cases.

判断数字的最小因数是不是只有2,3，5； 是True，不是false
数字1视为True

solution

class Solution(object):
    def isUgly(self, num):
        """
        :type num: int
        :rtype: bool
        """
        factors = [2,3,5]
        for i in factors:
            while  1:
                x, y = divmod(num,i)
                if y > 0:
                    break
                else:
                    num = x
        return num == 1
                    

直接除法运算， time limit exceeded

换一个思路，反过来，可以整除其他素数则返回False,问题转化为获取素数列表，求素数本身就是消耗时间的过程

discuss

        for p in 2, 3, 5:
            while num % p == 0 < num:
                num /= p
        return num == 1

为什么这个没有超时？

结果为True的时候，discuss 的效率没有更高，false的时候高一些

可能是多了一次判断

        if num <= 0:
            return False
        for x in [2, 3, 5]:
            while num % x == 0:
                num = num / x
        return num == 1