Excel Sheet Column Number

Spreadsheet columns are labelled A, B, ..., Z, AA, AB, .... Given a column title as a string of uppercase letters, return its corresponding 1-based column number.

So A -> 1, B -> 2, Z -> 26, AA -> 27, AB -> 28, and so on.

Input / output

• Input: columnTitle: string of uppercase letters A–Z
• Output: the column number as an integer

Examples

1. columnTitle = "A" returns 1.
2. columnTitle = "AB" returns 28.
3. columnTitle = "ZY" returns 701.

Constraints

• 1 <= columnTitle.length <= 7
• columnTitle contains only uppercase English letters.
• The value is in the range [1, 2^31 - 1].

Follow-up This is base-26 with digits 1..26 (there is no zero digit). Why does the absence of a zero digit make it bijective base-26 rather than ordinary base-26?