BAC is 2×(26^2) + 1×26 + 2 because we have visited all: xx + Axx (26^2) and x (26) and AB (2)

So first we find the last char: (num-1) % 26 to get the index

Then we divide n by 26 to become BA (2*26 + 1)

Then we get the last char and loop again

code:

```
class Solution(object):
def convertToTitle(self, n):
charList = list('ABCDEFGHIJKLMNOPQRSTUVWXYZ')
res = ''
while n > 0:
res += charList[(n-1) % 26]
n = (n-1) / 26 # let 26 become Z not AZ
return res[::-1]
```