Problem 39.4 · Math for Coding InterviewsEasy

Excel Sheet Column Number

What it teaches: Base 26 with digits 1–26.

Practise it on judges as “Excel Sheet Column Number”.

In plain words

Spreadsheet columns count A to Z, then AA, AB, … It is like counting in base 26 where A = 1 and Z = 26. Read the letters left to right; each new letter shifts the value so far by one place (× 26) and adds its own value, just like reading 3, then 37, then 375 in base 10.

Return the column number. Example: "ZY" → 701.

The problem

Convert a column title (A = 1, Z = 26, AA = 27, …) to its number.

Example 1

Input: columnTitle = "ZY"
Output: 701

Constraints

1 ≤ length ≤ 7

Pattern clues in the wording

→ Letters as digits

These clues point to Number Theory and Combinatorics: GCD, primes, modular arithmetic, fast power and counting formulas that turn loops into a few lines of math.

Stuck? Take one hint at a time

Solution · starter

class Solution {
    public int titleToNumber(String columnTitle) {
        return 0;
    }
}

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Test cases

#	Input	Expected
1	columnTitle = "A"	1
2	columnTitle = "AB"	28
3	columnTitle = "ZY"	701

From slow to fast

Approaches

1. Horner's rule in base 26 · O(n)

Horner's rule in base 26

Time O(n) Space O(1)

Read left to right, multiplying by 26 each step.

▶ Dry run: value = value × 26 + lettercolumnTitle = "ZY"

vars(vars)

value: 0

Step 1/3Start with 0.

Approach 1

class Solution {
    public int titleToNumber(String columnTitle) {
        int value = 0;
        for (char c : columnTitle.toCharArray()) value = value * 26 + (c - 'A' + 1);
        return value;
    }
}

Verdict: Same as parsing a decimal number.

Before you submit

Edge cases and common mistakes

Test these inputs

Single letter
"FXSHRXW" (the largest int-sized title)

Mistakes people make

Treating A as 0.

Interview

Follow-up questions

And the reverse (number to title)?

Connect the dots

Where this shows up in real systems

System Design · System 12.1 — URL ShortenerShort codes are numbers written in base 62, the same base conversion as Excel column titles (base 26).