LeetCode #13: Roman to Integer

🏛️ Roman Numerals Recap

Roman numerals use letters to represent values, with special cases for subtraction:

The key: if a smaller value appears before a larger one, it's subtracted.

The Problem

Given a valid Roman numeral string (from I to MMMCMXCIX), convert it to an integer.

Examples

✅ Algorithm: Left-to-Right Scan

We scan each character and:

1. Look up the value of the current symbol
2. If the current value is less than the next value, subtract it
3. Otherwise, add it
4. Move to the next symbol

Solution (JavaScript)

function romanToInt(s) {
  const map = {
    'I': 1,
    'V': 5,
    'X': 10,
    'L': 50,
    'C': 100,
    'D': 500,
    'M': 1000
  };

  let total = 0;

  for (let i = 0; i < s.length; i++) {
    const current = map[s[i]];
    const next = map[s[i + 1]] || 0;

    if (current < next) {
      total -= current;
    } else {
      total += current;
    }
  }

  return total;
}

🧠 Why This Works

Because Roman numerals are written in descending order — except for subtractive cases.

Whenever a smaller value appears before a larger one, it means subtraction. This algorithm catches that by comparing each symbol to the next.

🎮 Try It: Step-by-Step Decoder

Watch how each symbol is added or subtracted based on the next one.