4. ugly number ii

class  {
    /**
     * @param n an integer
     * @return the nth prime number as description.
     */
    public int nthUglyNumber(int n) {
        // 只有2，3，5 这三个factor
        int[] ugly = new int[n];
        int p2 = 0, p3 = 0, p5 = 0;
        ugly[0] = 1;
        for (int i = 1; i < n; i++) {
            ugly[i] = Math.min(ugly[p2] * 2, Math.min(ugly[p3] * 3, ugly[p5] * 5));
            // 有可能p2 * 2, p3 * 3, p5 * 5有一样的数，都要加一
            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];
    }
}