利用三个指针
https://leetcode-cn.com/problems/ugly-number-ii/solution/cchou-shu-by-fu-xi-bei-kao-de-long-long/

class Solution:
    def GetUglyNumber_Solution(self, index):
        # write code here
        if not index:
            return 0
        res = [1]
        idx2 = 0
        idx3 = 0
        idx5 = 0
        for i in range(index-1):
            res.append(min(res[idx2]*2,res[idx3]*3,res[idx5]*5))
            if res[-1] == res[idx2]*2:
                idx2 += 1
            if res[-1] == res[idx3]*3:
                idx3 += 1
            if res[-1] == res[idx5]*5:
                idx5 += 1
        return res[-1]