```
class Solution(object):
def convertToTitle(self, n):
"""
:type n: int
:rtype: str
"""
if n<=0:
return None
bound=ord('A')
dict={}
for i in range(1, 27, 1):
dict[i]=chr(i+bound-1)
rst=""
while n>0:
rst+=dict[n%26]
n/=26
return rst[::-1]
```