263.Ugly Number

Tags: Easy Math

Links: https://leetcode.com/problems/ugly-number/

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

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如果因数只有2，3，5，那么除以这些因子结果一定是1.

class Solution {
public:
    bool isUgly(int num) {
        std::ios_base::sync_with_stdio(false);
        cin.tie(NULL);
        cout.tie(NULL);

        if (num < 1) return false;

        while (num > 1 && num % 2 == 0) num /= 2;
        while (num > 1 && num % 3 == 0) num /= 3;
        while (num > 1 && num % 5 == 0) num /= 5;

        return num == 1; 
    }
};