编写程序寻找第n个“丑陋数” ugly number
丑陋数是指只包含质因子2, 3, 5的正整数。例如，1, 2, 3, 4, 5, 6, 8, 9, 10, 12 是前10个丑陋数。
注意，数字1也被视为丑陋数
class Solution(object):
def nthUglyNumber(self, n):
"""
:type n: int
:rtype: int
"""
# 6 star, 很巧妙的解法，多练几遍
a, b, c = 0, 0, 0
result = [1 for _ in range(n)]
for i in range(1, n):
result[i] = min(result[a]*2, result[b]*3, result[c]*5)
if result[i] == result[a]*2:
a += 1
if result[i] == result[b]*3:
b += 1
if result[i] == result[c] * 5:
c += 1
return result[n-1]