[Leetcode 264] Ugly Number II

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本文介绍了一种高效算法来找到序列中的第N个丑数。丑数定义为仅包含质因数2、3和5的正整数。文章通过使用有序数据结构如TreeSet来维护丑数集合，并逐步构建起完整的丑数序列。

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.

An ugly number must be multiplied by either 2, 3, or 5 from a smaller ugly number.

维护一个ordered data structure to store ugly number, every time pop first one.

public int nthUglyNumber(int n) {
        SortedSet<Long> s1 = new TreeSet<>();
        s1.add((long)1);
        long result = s1.first();
        for(int i=0;i<n;i++){
            result = s1.first();
            s1.add(result * 2);
            s1.add(result * 3);
            s1.add(result * 5);
            s1.remove(result);
        }
        return (int)result;
    }