丑数

把只包含因子2、3和5的数称作丑数（Ugly Number）。

例如6、8都是丑数，但14不是，因为它包含因子7。 习惯上我们把1当做是第一个丑数。求按从小到大的顺序的第N个丑数。

class Solution:
    def GetUglyNumber(self, index):
        if index == None or index <= 0:
            return None

        ugluNumber = [1] * index

        nextIndex = 1
        index2 = 0
        index3 = 0
        index5 = 0


        while nextIndex < index:
            minVal = min(ugluNumber[index2] * 2, ugluNumber[index3] * 3, ugluNumber[index5] * 5)
            ugluNumber[nextIndex] = minVal

            while ugluNumber[index2] * 2 <= ugluNumber[nextIndex]:
                index2 += 1
            while ugluNumber[index3] * 3 <= ugluNumber[nextIndex]:
                index3 += 1
            while ugluNumber[index5] * 5 <= ugluNumber[nextIndex]:
                index5 += 5

        return ugluNumber[-1]

package main

func UglyNum(nums int) int {
    res := make([]int, nums)

    if nums < 1 {return 0}

    index2, index3, index5 := 0, 0, 0
    index := 1
    res[0] = 1
    for index <= nums - 1 {
        minVal = min(min(res[index2] * 2, res[index3] * 3), res[index5] * 5)
        res[index] = minVal
        index++
        if minVal == res[index2] * 2{
            index2++
        }
        if minVal == res[index3] * 3 {
            index3++
        }
        if minVal == res[index5] * 5 {
            index5++
        }
    }
    return res[-1]
}

func min(a, b int) int {
    if a >= b {
        return a
    }
    return b
}