The Roman Converter

An easy Python interview practice problem on DataDriven. Write the Python and run it against test cases, with instant feedback.

Domain
Python
Difficulty
easy
Seniority
L3

Problem

Given a Roman numeral string using symbols I=1, V=5, X=10, L=50, C=100, D=500, M=1000, return its integer value. When a smaller symbol precedes a larger one (IV, IX, XL, XC, CD, CM), subtract it from the larger one.

Roman numerals decoded.

Example

Input:

s = "MCMXCIV"

Output:

1994

Worked solution and explanation

Why this problem exists in real interviews

This tests pattern recognition with lookup tables and the subtraction rule in Roman numerals. It checks whether candidates can handle the exception case (IV, IX, XL, etc.) elegantly within a left-to-right scan.

Break down the requirements

Step 1: Build a symbol-to-value mapping

Map each Roman numeral character to its integer value: I=1, V=5, X=10, L=50, C=100, D=500, M=1000.

Step 2: Scan left to right

For each character, look up its value and add it to the total.

Step 3: Handle the subtraction rule

If the current value is less than the next value, subtract it instead of adding. This handles cases like IV (4) and IX (9).

The solution

Left-to-right scan with subtraction lookahead
def roman_to_int(s):
    values = {'I': 1, 'V': 5, 'X': 10, 'L': 50, 'C': 100, 'D': 500, 'M': 1000}
    total = 0
    for i in range(len(s)):
        current = values[s[i]]
        if i + 1 < len(s) and current < values[s[i + 1]]:
            total -= current
        else:
            total += current
    return total

Common follow-up questions