LeetCode:《Excel Sheet Column Title》

编程思路：类似于26进制，但是又不是，26进制每位可以取值0-25，而此题取值1-26，所以处理的时候需要进行调整，以免出现零

class Solution:
# @return a string
def convertToTitle(self, num):
li = list()
d = {1:'A', 2:'B', 3:'C', 4:'D', 5:'E', 6:'F', 7:'G', 8:'H', 9:'I',
10:'J',11:'K', 12:'L', 13:'M', 14:'N', 15:'O', 16:'P', 17:'Q',18:'R',
19:'S', 20:'T', 21:'U', 22:'V', 23:'W', 24:'X', 25:'Y', 26:'Z'}

# 确定一共多少个字母
a = math.log(25*num+26) / math.log(26)
b = math.log(25*num+26) // math.log(26)
n = int(a - 1) if a == b else int(a)
i = n
while i > 0:
value = int(num / math.pow(26, i-1))
num = num - math.pow(26, i-1)*value
# 如果还未确定后面的各个位置的字母，num变为0，需调整
if num == 0 and i != 1:
value = value - 1
num = num + math.pow(26, i-1)
li.append(d[value])
i = i - 1
return ''.join(li)