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85. Excel Sheet Column Number

mediumAsked at Vercel

Convert an Excel-style column title (A, B, ..., Z, AA, AB, ...) to its corresponding integer. It looks like a one-liner — and the loop is — but the question hides a real numeral-system subtlety: this is bijective base-26, a system with digits 1 through 26 and no zero. Vercel candidates report it as a screen warm-up where saying the word 'bijective' (or at least explaining why there is no zero digit) is the entire differentiator.

By Sam K., Founder, InterviewChamp.AI · Last verified

Source citations

Public interview reports confirming this problem appears in Vercel loops.

  • Glassdoor (2025-12)— Vercel screen warm-up.
  • LeetCode Discuss (2026-Q1)— Listed in Vercel new-grad screen recap.

Problem

Given a string columnTitle that represents the column title as appears in an Excel sheet, return its corresponding column number. A maps to 1, Z to 26, AA to 27, AB to 28 — the lettering continues like a numeral system that has no zero digit.

Constraints

  • 1 <= columnTitle.length <= 7
  • columnTitle consists only of uppercase English letters.
  • columnTitle is in the range ['A', 'FXSHRXW'].

Examples

Example 1

Input
columnTitle = "A"
Output
1

Explanation: Single letters map directly: A is the 1st column. There is no letter for zero — the first hint this is not standard base-26.

Example 2

Input
columnTitle = "AB"
Output
28

Explanation: A contributes 1 * 26 = 26, B contributes 2: total 28. Note AA is 27, immediately after Z at 26 — the sequence never 'resets' through a zero.

Example 3

Input
columnTitle = "ZY"
Output
701

Explanation: Z is 26, Y is 25: 26 * 26 + 25 = 701. Multi-letter titles accumulate exactly like digits, just offset by one.

Approaches

1. Recursive base-26

title -> base26 number, like converting a decimal string to int but with base 26 and A=1.

Time
O(n)
Space
O(n) stack
function titleToNumber(columnTitle) {
  if (columnTitle.length === 0) return 0;
  const last = columnTitle.charCodeAt(columnTitle.length - 1) - 64;
  return titleToNumber(columnTitle.slice(0, -1)) * 26 + last;
}

Tradeoff: Identical arithmetic to the loop but pays O(n) call stack and string slicing for no benefit — useful only to show the recurrence value(s) = value(prefix) * 26 + digit before collapsing it into the iterative form.

2. Iterative left-to-right base-26 (optimal)

Walk the string; result = result * 26 + (char - 'A' + 1).

Time
O(n)
Space
O(1)
function titleToNumber(columnTitle) {
  let n = 0;
  for (const c of columnTitle) {
    n = n * 26 + (c.charCodeAt(0) - 64); // 'A' = 65, so 'A' - 64 = 1
  }
  return n;
}

Tradeoff: Single pass, O(1) space. The key insight: this is base-26 BUT with digits 1-26 instead of 0-25 (no 'zero' character).

Vercel-specific tips

Vercel grades the base-26-with-offset insight. Bonus signal: pointing out that this is bijective base-26 (no zero character) and contrasting with standard base-26 where AA would equal A. Mention the inverse problem (LC 168) where the off-by-one shows up again.

Common mistakes

  • Using `c.charCodeAt(0) - 65` — that gives 0 for A, breaking the 1-indexing.
  • Treating it as standard base-26 — AA would be 26, not 27.
  • Forgetting to multiply by 26 before adding the new digit.

Follow-up questions

An interviewer at Vercel may pivot to one of these next:

  • Excel Sheet Column Title (LC 168) — the inverse.
  • Generalize to any custom alphabet.
  • Base conversion in arbitrary bases.

Solve it now

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Output

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FAQ

Why -64 and not -65?

'A' should map to 1, not 0. The character code of 'A' is 65, so 65 - 64 = 1. This is bijective base-26.

Why isn't this standard base-26?

Because there's no 'zero' character. AA != A; AA = 27. Standard base-26 would let AA == A by leading-zero suppression.

Can the result overflow?

Not within the stated constraints — the maximum title FXSHRXW is 2,147,483,647, exactly the 32-bit integer ceiling, and JavaScript numbers handle it natively. In a fixed-width language, mention that the constraint was chosen to sit at INT_MAX on purpose.

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