263. Ugly Number

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. 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.

链接： http://leetcode.com/problems/ugly-number/

题解：

验证丑数。数学题，我们直接除就好了。

Time Compleixty - O(logn)， Space Complexity - O(1).

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

 

二刷：

Java:

Time Compleixty - O(logn)， Space Complexity - O(1).

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

 

学习Stefan的写法

public class Solution {
    public boolean isUgly(int num) {
       for (int i = 2; i < 6 && num != 0; i++) {
           while (num % i == 0) {
               num /= i;
           }
       }
       return num == 1;
    }
}

 

三刷:

Java:

Time Compleixty - O(logn)， Space Complexity - O(1).

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

 

Reference:

https://leetcode.com/discuss/52703/2-4-lines-every-language