The O(n) Club: Excel Sheet Column Number – Crack Excel’s Letter-to-Number Code, Finally

The O(n) Club: Excel Sheet Column Number – Crack Excel’s Letter-to-Number Code, Finally

⚡ TL;DR

This problem: turn Excel’s column letters (A, AB, ZY, etc.) into their actual numbers. It’s base-26, but not the zero-based variety. You go left-to-right, multiply by 26, and for once, treat ‘A’ as 1 rather than giving yourself another off-by-one PTSD episode. Here’s how you’d hack it in Java:

// Java magic for Excel columns
total = 0;
for (char c : columnTitle.toCharArray()) {
    total = total * 26 + (c - 'A' + 1);
}

🧠 How Devs Usually Mess This Up

Tempted to start at zero? Welcome to Excel’s first trap. Most devs default to zero-indexing (‘A’ == 0), since that’s just muscle memory by now. But Excel? ‘A’ is 1, sorry. And parsing right-to-left, like you’re reading a power-of-two table, just gets you kicked by the order. Oh—and if you think ‘AA’ is 26, try again. It’s 27. Excel laughs while you debug.

🔍 What’s Actually Going On

Think of Excel as running its own ancient civilization, where numerals are forbidden and everything is alphabetical. Counting goes: ‘A’, ‘B’, …, ‘Z’, then ‘AA’, ‘AB’, and so on. This system is base-26, but there’s no zero digit. Each letter’s value is what it would be on a Scrabble board (‘A’=1, ‘B’=2, …, ‘Z’=26). Leftmost letter? Give it the most weight (think of the ‘hundreds’ place in decimal). Multiply your total by 26 for each new leftward character, then add the letter’s value. Wonder why ‘ZY’ is 701? Because (26*26=676), plus (25), gives you 701—algebra, but with more letters and less dread.

🛠️ PseudoCode

• Set your total to 0.
  int total = 0;
  This number will bear the brunt of all Excel logic.
• Loop left-to-right through every character.
  for (char c : columnTitle.toCharArray())
  It reads English-style, not binary wonk.
• Convert each character from ‘A’-‘Z’ into 1..26.
  int value = c - 'A' + 1;
  So, ‘A’ gives 1, not 0—no exceptions, not even for Z.
• Scale the previous total and add value.
  total = total * 26 + value;
  Multiply by 26 to shift left, then add this character’s value. Repeat for each character.
• After the loop, return total like a champion.
  return total;
  Smile. Spreadsheet world domination is yours.

💻 The Code

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

⚠️ Pitfalls, Traps & Runtime Smacks

• Zero-indexing pain: If you treat ‘A’ as 0, you’ll get answers that only make sense to spreadsheet goblins.
• Lowercase blues: Input is uppercase. If it isn’t, convert it, or you’ll have ASCII math gone wild.
• Overflow drama: Unless you’re parsing columns well past ‘ZZZZ’, Java int is safe enough. (If not, use long and accept your fate.)
• Parse direction: Go left-to-right. Your intuition will try to trick you. Don’t listen.
• Runtime reality: O(N) time, N = length of input, O(1) space. No need to summon extra RAM.

🧠 Interviewer Brain-Tattoo

“Excel columns: Where ‘AA’ follows ‘Z,’ and the only zero you see is your will to live after debugging off-by-one errors.”