PPractick

The problem

A museum's catalogue lists exhibit years in Roman numerals, like MCMXCIV, and the search system needs them as integers. Roman numerals mostly add up symbol values (VIII = 5 + 1 + 1 + 1), with one twist: a smaller symbol before a larger one is subtracted, as in IV = 4, IX = 9 and CM = 900. So the conversion is a single left-to-right pass, subtracting a symbol when the next one is larger and adding it otherwise.

Example: "MCMXCIV" → 1994 (M + CM + XC + IV = 1000 + 900 + 90 + 4).

Function signature

int romanToInt(const string& s);  // valid Roman numeral in [1, 3999]

Test cases

Input s	Output	Explanation
"III"	3
"LVIII"	58	L + V + III = 50 + 5 + 3
"MCMXCIV"	1994	M + CM + XC + IV
"IV"	4	Subtractive form
"IX"	9
"MMMCMXCIX"	3999	The largest standard numeral

(Confirm: the input is a valid standard numeral using I, V, X, L, C, D, M; subtractive pairs are IV, IX, XL, XC, CD and CM.)

How interviewers phrase it

• "Convert a Roman numeral to an integer" (LeetCode 13).
• The reverse direction: integer to Roman (LeetCode 12), a greedy problem.

Clarifying questions to ask

1. Is the input guaranteed to be valid?
2. What range? (Standard numerals go up to 3999.)
3. Uppercase only?

Follow-ups to expect

1. "How do you detect the subtractive cases?"
2. "Why does comparing with the next symbol work for all six subtractive pairs?"
3. "How do you convert an integer to Roman?"
4. "How would you validate the input?"
5. "Complexity?"

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