JZ33丑数题的另一种思路

大都是三指针解法，分享一种容易理解的思路吧，很好理解，懒得讲了。

# -*- coding:utf-8 -*-
class Solution:
    def GetUglyNumber_Solution(self, index):
        # write code here
        if index <= 0:
            return 0
        res = {1}
        cur = 1
        for i in range(index):
            cur = min(res)
            res.remove(cur)
            res.add(cur * 2)
            res.add(cur * 3)
            res.add(cur * 5)
        return cur