[LeetCode每日一题]264. Ugly Number II

题目如下：

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

Ugly numbers are positive numbers whose prime factors only include 2, 3, 5. 

Example:
Input: n = 10
Output: 12
Explanation: 1, 2, 3, 4, 5, 6, 8, 9, 10, 12 is the sequence of the first 10 ugly numbers.

Note:
1.1 is typically treated as an ugly number.
2.n does not exceed 1690.

264. Ugly Number II 这道题让我们找出第n大的ugly数，ugly数是什么请看[LeetCode每日一题]263. Ugly Number。假设初始时ugly数只有2，3，5，新生成的ugly数都是由原来的ugly数乘2, 乘3, 乘5得来，由此便可以写出代码。

class Solution {
    
    public int nthUglyNumber(int n) {
        if (n <= 0 || n > 1690) return -1;
        int[] uglys = new int[n];
        uglys[0] = 1;
        int p2 = 0, p3 = 0, p5 = 0;
        for (int i = 1; i < n; i++) {
            uglys[i] = Math.min(uglys[p2] * 2, Math.min(uglys[p3] * 3, uglys[p5] * 5));
            if (uglys[p2] * 2 == uglys[i]) p2++;
            if (uglys[p3] * 3 == uglys[i]) p3++;
            if (uglys[p5] * 5 == uglys[i]) p5++;
        }
        return uglys[n - 1];
    }
}