# 171. Excel Sheet Column Number

#### Easy

***

Given a string `columnTitle` that represents the column title as appear in an Excel sheet, return *its corresponding column number*.

For example:

```
A -> 1
B -> 2
C -> 3
...
Z -> 26
AA -> 27
AB -> 28 
...
```

 

**Example 1:**

```
Input: columnTitle = "A"
Output: 1
```

**Example 2:**

```
Input: columnTitle = "AB"
Output: 28
```

**Example 3:**

```
Input: columnTitle = "ZY"
Output: 701
```

 

**Constraints:**

* `1 <= columnTitle.length <= 7`
* `columnTitle` consists only of uppercase English letters.
* `columnTitle` is in the range `["A", "FXSHRXW"]`.

```python
class Solution:
    def titleToNumber(self, columnTitle: str) -> int:
        length = len(columnTitle)
        index = 1
        col_num = 0
        for char in list(columnTitle):
            c_val = ord(char) - ord('A') + 1
            col_num += pow(26, length - index)*c_val
            index += 1
        return col_num
```