Python 丑数 | 码农家园

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，因为它包含质因子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。

方法一，运行速率慢，当N较大时尤为明显：

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# -*-coding:utf-8 -*-
class Solution:
def GetUglyNumber_Solution(self, index):
if index < 1:
return None

count = 0

def isUglyNumber(num):
while num % 2 == 0:
num = num / 2
while num % 3 == 0:
num = num / 3
while num % 5 == 0:
num = num / 5
if num == 1:
return True
else:
return False

num = 1

while True:
if isUglyNumber(num):
count += 1
if count == index:
return num
num += 1

if __name__ == '__main__':
s = Solution()
ret = s.GetUglyNumber_Solution(10)
print(ret)

运行结果为:

方法二，运行速度快，应该采用该方法:

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# -*-coding:utf-8 -*-
class Solution:
def GetUglyNumber_Solution(self, index):
if index < 1:
return 0

uglyList = [1]
twoPointer = 0
threePointer = 0
fivePointer = 0
count = 1
while count != index:
minValue = min(2 * uglyList[twoPointer], 3 * uglyList[threePointer], 5 * uglyList[fivePointer])
uglyList.append(minValue)
count += 1
if minValue == 2 * uglyList[twoPointer]:
twoPointer += 1

if minValue == 3 * uglyList[threePointer]:
threePointer += 1

if minValue == 5 * uglyList[fivePointer]:
fivePointer += 1
return uglyList[count - 1]

if __name__ == '__main__':
s = Solution()
ret = s.GetUglyNumber_Solution(200)
print(ret)

运行结果为：

16200