Date: 2026-06-06
Time: 18:57
roman-to-integer/solution.pyThis file implements LeetCode problem #13 — Roman to Integer. It owns a single responsibility: converting a valid Roman numeral string (like "MCMXCIV") into its integer equivalent (1994). It's a standalone solution module following the repo's convention of one problem per directory.
romantoint(s: str) -> int — The sole public function. Contract: given a string s containing only valid Roman numeral characters representing a value in [1, 3999], returns the corresponding integer.
values dict — Maps the seven Roman numeral symbols to their integer values. Defined locally inside the function (not module-level), so it's reconstructed on each call.
The solution uses the subtraction-rule scan pattern: iterate left to right, and at each position decide whether to add or subtract the current symbol's value based on a lookahead comparison with the next symbol.
This is the canonical single-pass approach for this problem. It exploits the fact that in valid Roman numerals, a smaller value preceding a larger value always means subtraction (e.g., IV = 4, XC = 90). In all other cases, values are additive.
The iteration uses index-based range(len(s)) rather than enumerate or zip — slightly more verbose but makes the i + 1 lookahead bounds check explicit.
Imports: None. Pure Python, no standard library usage.
Imported by: The roman-to-integer/test_solution.py file directly. The massive "Imported By" list in the prompt is misleading — those are unrelated test files across the repo, likely an artifact of the analysis tool matching on a shared test runner or import pattern, not actual imports of this function.
1. Build the values lookup table.
2. Initialize result = 0.
3. For each index i in the string:
i + 1 is in bounds and values[s[i]] < values[s[i + 1]], this is a subtraction case (e.g., I before V). Subtract values[s[i]] from result.values[s[i]] to result.4. Return result.
For input "XIV":
i=0: X(10) vs I(1) → 10 >= 1 → add 10. Result: 10i=1: I(1) vs V(5) → 1 < 5 → subtract 1. Result: 9i=2: V(5), no next → add 5. Result: 14s contains only valid Roman characters or represents a well-formed numeral. A KeyError would propagate on invalid characters.None. Invalid characters raise KeyError from the values dict lookup. Empty string input returns 0 (the loop body never executes). There's no explicit validation — the function trusts the caller to provide valid input per the LeetCode problem constraints.
roman-to-integer/test_solution.py — See what edge cases the test suite covers (empty string, single characters, max value 3999)roman-to-integer/review.md — The code review likely discusses alternative approaches and complexity analysisroman-to-integer/plan.md — Design decisions made before implementationsubtraction-rule-alternatives — Compare this lookahead approach with the reverse-iteration variant (iterate right-to-left, track previous value) or the two-character prefix matching approachexcel-sheet-column-number/solution.py:titleToNumber — Similar positional-value accumulation pattern applied to base-26 conversionroman-subtraction-lookahead — romantoint subtracts values[s[i]] if and only if values[s[i]] < values[s[i+1]], which correctly handles all six Roman subtraction pairs (IV, IX, XL, XC, CD, CM)roman-single-pass-linear — The function performs exactly one pass over the input string with O(1) work per character, giving O(n) time and O(1) spaceroman-no-input-validation — The function assumes valid input; invalid characters will raise an unhandled KeyError from the values dictroman-empty-returns-zero — An empty string input returns 0 because the loop doesn't execute and result is initialized to 0