[LeetCode] 171. Excel Sheet Column Number

🧠 Remember: “Base-26 with no zero — A is 1, not 0.”

The problem — 20 seconds

"A" → 1, "AB" → 28, "ZY" → 701.

Convert an Excel column title to its number — base-26 with digits 1–26 instead of 0–25. That one shift is the entire problem.

Original on LeetCode

"AA" → 1 × 26 + 1 = 27
"ZY" → 26 × 26 + 25 = 701

That’s the whole algorithm. Everything below just makes it obvious why.

Your first instinct

Add 26 ** i × value per position from the right via a lookup dict. Correct — but powers, reversed(), and a dict are ceremony.

The turning point

Read left → right and fold: n = n × 26 + digit. Each letter shifts everything so far one place left and appends its value — Horner’s rule, the decimal-parsing trick. ord(ch) − ord('A') + 1 is the digit; no dict needed.

Trace "ZY":

n = 0 → 'Z' → 0 × 26 + 26 = 26 → 'Y' → 26 × 26 + 25 = 701  ✓

What the code is really saying

class Solution:
    def titleToNumber(self, s: str) -> int:
        n = 0
        for ch in s:
            n = n * 26 + (ord(ch) - ord("A") + 1)
        return n

Shift-and-add, one character at a time. One trap: treating A as 0 makes "AA" give 0 — the + 1 is the “no zero digit” insight.

Why it works

Invariant: after k characters, n equals that prefix’s value. Appending a letter shifts it one column left and adds the digit — positional notation. One pass, one accumulator: time O(n), space O(1).

🧠 Remember

Excel Column → “Base-26 with no zero.” Fold left to right: n = n × 26 + digit, digits starting at 1.

🔁 Try this: invert it — number → title (LeetCode 168). (Hint: subtract 1 before each divmod, or 26 gives AZ instead of Z.)

Related: Roman to Integer · Reverse String