171. Excel Sheet Column Number

Problem¶

Convert an Excel column label to its 1-based column index.

A -> 1
Z -> 26
AA -> 27

Example:

input:  "AB"
state:  A=1, B=2 -> 1*26 + 2 = 28
output: 28

Key trick¶

Treat the string like a base-26 number, but with digits A..Z mapped to 1..26 instead of 0..25.

Trap¶

The main mistake is using normal base-26 logic with A = 0, which gives wrong results for labels like A and AA.

Why is it interesting?¶

It tests whether you can recognize a custom positional numeral system and convert it cleanly in linear time.

Python solution¶

class Solution:
    def titleToNumber(self, columnTitle: str) -> int:
        total = 0

        # Standard left-to-right base-26 accumulation.
        for ch in columnTitle:
            total = total * 26 + (ord(ch) - ord("A") + 1)

        return total

Comment on my solution¶

Your solution is correct.

It computes the value from right to left using powers of 26.
It has time complexity \(O(n)\) and space complexity \(O(1)\).
A slightly more idiomatic version is to accumulate left to right, which avoids repeated exponentiation.

## Solution

class Solution:
    def titleToNumber(self, columnTitle: str) -> int:
        total = 0
        n = len(columnTitle)
        for i in range(n):
            ch = columnTitle[n - 1 - i]
            digit = ord(ch) - ord("A") + 1
            total += digit * (26**i)

        return total


Solution().titleToNumber("A") # 1
Solution().titleToNumber("B") # 2
Solution().titleToNumber("AB") # 28
Solution().titleToNumber("ZY") # 701