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 = 10Output: 12Explanation: 1, 2, 3, 4, 5, 6, 8, 9, 10, 12 is the sequence of the first 10 ugly numbers.
Note:
1 is typically treated as an ugly number.n does not exceed 1690.难度：medium
题目：写程序找出第n个丑数。丑数公因子只由2,3,5构正的正整数。
思路：三路指针。
Runtime: 2 ms, faster than 99.79% of Java online submissions for Ugly Number II.
Memory Usage: 36.8 MB, less than 32.39% of Java online submissions for Ugly Number II.class Solution { public int nthUglyNumber(int n) { int[] ugly = new int[n]; ugly[0] = 1; int p2 = 0, p3 = 0, p5 = 0; for (int i = 1; i < n; i++) { ugly[i] = Math.min(ugly[p2] * 2, Math.min(ugly[p3] * 3, ugly[p5] * 5)); if (ugly[i] == ugly[p2] * 2) { p2++; } if (ugly[i] == ugly[p3] * 3) { p3++; } if (ugly[i] == ugly[p5] * 5) { p5++; } } return ugly[n - 1]; }}