[Leetcode 264] Ugly Number II

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.

1. 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;
    }