Convert a number to an Excel column, and convert an Excel column back to a number.
In Excel, columns are labeled as:
- 1 →
"A" - 2 →
"B" - …
- 26 →
"Z" - 27 →
"AA" - 28 →
"AB" - …
You are asked to implement two functions:
- Given a positive integer, convert it to its corresponding Excel column title (e.g.,
1 -> "A",28 -> "AB"). - Given an Excel column title (string of uppercase letters), convert it back to its corresponding number (e.g.,
"A" -> 1,"AB" -> 28).
You may assume:
- The integer input is positive
- The column title consists of only uppercase letters
A–Z
- Number → Column: treat it as a 26-base system without zero; repeatedly do
n--, take(n % 26)to map0..25→A..Z, thenn /= 26. - Column → Number: scan from left to right, accumulate like normal base-26:
res = res * 26 + (ch - 'A' + 1).
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