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"]`.