Excel Sheet Column Number / Title (base conversion).

Problem. Convert between Excel column titles and column numbers:

• Title → Number: "A" → 1, "Z" → 26, "AA" → 27, "AB" → 28.
• Number → Title: the inverse.

This is base-26 conversion, but with a twist: it's 1-indexed ("A" = 1, not 0), so there's no digit for zero.

Title to number — straight base-26 evaluation

Treat the title as a base-26 number where A..Z map to 1..26. Horner-style accumulate:

int titleToNumber(String s) {
    int result = 0;
    for (int i = 0; i < s.length(); i++) {
        int digit = s.charAt(i) - 'A' + 1;   // 'A' -> 1 ... 'Z' -> 26
        result = result * 26 + digit;        // shift left one base-26 place, add digit
    }
    return result;
}

"AB" = 1 * 26 + 2 = 28. This direction is the easy one — ordinary positional evaluation.

Number to title — the bijective-base-26 trick

Normal base conversion has digits 0..25. Here digits are 1..26 with no zero, so before each % 26 you must subtract 1 to shift into 0..25, then map back by adding 'A':

String convertToTitle(int n) {
    StringBuilder sb = new StringBuilder();
    while (n > 0) {
        n--;                                  // shift to 0-indexed: critical step
        sb.append((char) ('A' + n % 26));     // current least-significant "digit"
        n /= 26;
    }
    return sb.reverse().toString();
}

The n-- is the whole problem. Without it, 26 would map to "A0"-style garbage; with it, 26 → "Z" and 27 → "AA" come out right. This is sometimes called bijective base-26 — every positive integer maps to a unique non-empty string and vice versa, with no leading-zero ambiguity.

Walkthrough — 28 → "AB"

n = 28: n-- -> 27; 27 % 26 = 1 -> 'B'; n = 27/26 = 1
n = 1 : n-- -> 0;   0 % 26 = 0 -> 'A'; n = 0/26 = 0
reverse("BA") -> "AB"   ✓