**[Lintcode]Ugly Number II 丑数 II

Ugly number is a number that only have factors 2, 3 and 5.

Design an algorithm to find the nth ugly number. The first 10 ugly numbers are 1, 2, 3, 4, 5, 6, 8, 9, 10, 12...

分析：

(1) 1×2, 2×2, 3×2, 4×2, 5×2, …

(2) 1×3, 2×3, 3×3, 4×3, 5×3, …

(3) 1×5, 2×5, 3×5, 4×5, 5×5, …

分成三组，每次取最小值做丑数，每组使用一个指针。然后被选中的数的指针后移。

class Solution {    /**     * @param n an integer     * @return the nth prime number as description.     */    public int nthUglyNumber(int n) {        int[] arr = new int[n];        arr[0] = 1;        int index = 1;        int a = 2, b = 3, c = 5;        int indexA = 1, indexB = 1, indexC = 1;        for(int i = 1; i < arr.length; i++) {            int min = Math.min(Math.min(a, b), c);            arr[i] = min;                        if(a == min) a = arr[indexA++] * 2;            if(b == min) b = arr[indexB++] * 3;            if(c == min) c = arr[indexC++] * 5;        }        return arr[arr.length - 1];    }};