Problem:

Write a program to find the n-th ugly number.

Ugly numbers are positive numbers whose prime factors only include 2, 3, 5. For example, 1, 2, 3, 4, 5, 6, 8, 9, 10, 12 is the sequence of the first 10 ugly numbers.

Note that 1 is typically treated as an ugly number, and n does not exceed 1690.

Solution:

C++ / O(n)

// Author: huahua
// Running time: 6 ms
class Solution {
public:
    int nthUglyNumber(int n) {
        static vector<int> nums{1};
        static int i2 = 0;
        static int i3 = 0;
        static int i5 = 0;
        while (nums.size() < n) {
            const int next2 = nums[i2] * 2;
            const int next3 = nums[i3] * 3;
            const int next5 = nums[i5] * 5;
            const int next = min(next2, min(next3, next5));
            if (next == next2) ++i2;
            if (next == next3) ++i3;
            if (next == next5) ++i5;
            nums.push_back(next);
        }
        return nums[n - 1];
    }
};

C++ /query * O(NlogN)

// Author: Huahua
// Running time: 89 ms
class Solution {
public:
    int nthUglyNumber(int n) {
        vector<int> nums;
        for (long a = 1; a <= INT_MAX; a *= 2)
            for (long b = a; b <= INT_MAX; b *= 3)
                for (long c = b; c <= INT_MAX; c *= 5)
                    nums.push_back(c);
        std::sort(nums.begin(), nums.end());        
        return nums[n - 1];
    }
};

C++ / static variables O(NlogN) + query*O(1)

// Author: Huahua
// Running time: 7 ms
class Solution {
public:
    int nthUglyNumber(int n) {
        static vector<int> nums;
        if (nums.empty()) {
            for (long a = 1; a <= INT_MAX; a *= 2)
                for (long b = a; b <= INT_MAX; b *= 3)
                    for (long c = b; c <= INT_MAX; c *= 5)
                        nums.push_back(c);
            std::sort(nums.begin(), nums.end());
        }
        return nums[n - 1];
    }
};

C++ / table O(1)
leetcode_264_table.cc

Related Problems:

• 花花酱 LeetCode 263. Ugly Number

The post 花花酱 LeetCode 264. Ugly Number II appeared first on Huahua's Tech Road.

题目大意：如果一个数的素数因子只有2，3，5，我们称它为丑数。让你判断一个数是不是丑数。

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.

Idea:

Math

Solution:

C++

// Author: Huahua
// Running time: 9 ms
class Solution {
public:
    bool isUgly(int num) {
        const vector<int> factors{2, 3, 5};
        for (const int factor : factors)
            while (num && num % factor == 0) num /= factor;
        return num == 1;
    }
};

Related Problems:

• 花花酱 LeetCode 264. Ugly Number II

The post 花花酱 LeetCode 263. Ugly Number appeared first on Huahua's Tech Road.