Time: O(n) where n is the length of input string

Space: O(1)

```
class Solution(object):
def titleToNumber(self, s):
"""
:type s: str
:rtype: int
"""
# time: O(n)
# space: O(1) - constant
hashmap = {
'A': 1,
'B': 2,
'C': 3,
'D': 4,
'E': 5,
'F': 6,
'G': 7,
'H': 8,
'I': 9,
'J': 10,
'K': 11,
'L': 12,
'M': 13,
'N': 14,
'O': 15,
'P': 16,
'Q': 17,
'R': 18,
'S': 19,
'T': 20,
'U': 21,
'V': 22,
'W': 23,
'X': 24,
'Y': 25,
'Z': 26
}
res = 0
length = len(s)
for i in range(length):
res += hashmap[s[i]] * (26 ** (length - 1 - i))
return res
```