Excel column number from column title

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

column column number

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

Examples:

Input: A
Output: 1
Explanation: A is the first column so the output is 1.

Input: AA
Output: 27
Explanation: The columns are in order A, B, ..., Y, Z, AA .. So, there are 26 columns after which AA comes.

Base-26 to Decimal - O(n) Time and O(1) Space

The process is similar to binary to decimal conversion. Treat the Excel column title as a base-26 number, where A = 1, B = 2, ..., Z = 26.  For example, to convert AB, the formula is 26 * 1 + 2.

To convert CDA,
3*26*26 + 4*26 + 1
= 26(3*26 + 4) + 1
= 26(0*26 + 3*26 + 4) + 1

Take the input as string and the traverse the input string from the left to right and calculate the result as follows: 

result = 26*result + s[i] - 'A' + 1

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