C++ Algorithm to Check Ugly Number

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.

Naive Solution – Simulation

Divide the number by factor 2, 3 or 5 until it cannot be done. If the remainder is 1, then it is ugly number.

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

A better, shorter version could be done by only checking the 2, 3 and 5 factors: Algorithms to Determine a Ugly Number/Integer

See also: Teaching Kids Programming – Ugly Number Detection Algorithm

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