Problem description:

Given a column title as appear in an Excel sheet, return its corresponding column number.

For example:

1
2
3
4
5
6
7
8
A -> 1
B -> 2
C -> 3
...
Z -> 26
AA -> 27
AB -> 28 
...

Example 1:

1
2
Input: "A"
Output: 1

Example 2:

1
2
Input: "AB"
Output: 28

Example 3:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
Input: "ZY"
Output: 701
``` 

Constraints:

`1 <= s.length <= 7`
`s` consists only of uppercase English letters.
`s` is between "A" and "FXSHRXW".### Solution:


### Solution:


```python
class Solution:
    def titleToNumber(self, s: str) -> int:
        s = s[::-1]
        res = 0
        for exp, char in enumerate(s):
            res += 26**exp * (ord(char)-ord('A')+1)
        return res

time complexity: $O(n)$
space complexity: $O(1)$
reference:
related problem: