Math & Number Theory - Excel Sheet Column Title

Given an integer columnNumber, return its corresponding column title as it appears in an Excel sheet. For example: 1 → "A", 2 → "B", ..., 26 → "Z", 27 → "AA", 28 → "AB", ..., 701 → "ZY".

Input: columnNumber = 1
Output: "A"

Input: columnNumber = 28
Output: "AB"
Explanation: 28 = 1*26 + 2, so the title is "AB".

Input: columnNumber = 701
Output: "ZY"

This is a decimal-to-base-26 conversion, but the alphabet is 1-indexed (A=1 through Z=26) with no zero digit. The key trick is to decrement columnNumber by 1 before each modulo operation to shift to 0-indexed temporarily.

While columnNumber > 0: decrement by 1, then get remainder (columnNumber - 1) % 26.
Prepend the character 'A' + remainder to the result.
Divide: columnNumber = (columnNumber - 1) / 26.
Reverse the accumulated string and return.

class Solution { public String convertToTitle(int columnNumber) { StringBuilder sb = new StringBuilder(); while (columnNumber > 0) { columnNumber--; sb.append((char) ('A' + columnNumber % 26)); columnNumber /= 26; } return sb.reverse().toString(); } }

Complexity Analysis:

Time complexity: O(log₂₆ n) — number of characters in the result.
Space complexity: O(log₂₆ n) — space for the result string.