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NeetCode #881LC-168EasyMath & GeometryNC 250
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#881 · #168 · Excel Sheet Column Title(Excel 表列名称)

📌 Problem Statement & Constraints

Given an integer columnNumber, return its corresponding column title as it appears in Excel, where column 1 is A, column 26 is Z, column 27 is AA, and so on. Constraints: 1 <= columnNumber <= 2^31 - 1.

💡 Core Algorithmic Approaches

  1. Excel titles use bijective base-26: there is no zero digit, so A represents 1 rather than 0.
  2. Subtract one before taking the remainder; this converts the representation into an ordinary base-26 with digits 0..25.
  3. Map the remainder to a letter, append it, then divide by 26 and repeat.
  4. Because the least significant digit is produced first, reverse the collected letters at the end.

💻 Benchmark Python3 Implementation

class Solution:
    def convertToTitle(self, columnNumber: int) -> str:
        res = []
        while columnNumber > 0:
            columnNumber -= 1                       # shift to zero-based
            res.append(chr(ord("A") + columnNumber % 26))
            columnNumber //= 26
        return "".join(reversed(res))

⚡ Complexity Deep Dive

⏱️ Time Complexity
O(log_26 n): the value shrinks by a factor of 26 each iteration.
💾 Space Complexity
O(log_26 n): the output string.

⚠️ Interview Pitfalls & Follow-ups

  • Using plain base-26 without the - 1: 26 would become Z plus a carry, producing AZ instead of Z.
  • Taking the remainder before subtracting one: every non-leading digit would be off by one.
  • Building the string left to right: digits are generated least-significant first, so a reversal is needed.
  • Special-casing columnNumber % 26 == 0: the - 1 trick handles that case uniformly and avoids branching.