> ## Documentation Index
> Fetch the complete documentation index at: https://leetcode-py.wisl.dev/llms.txt
> Use this file to discover all available pages before exploring further.

> ## Agent Instructions
> leetcode-py is a Python LeetCode practice environment generator with one CLI: lcpy. It is not a service or platform.
> Each problem is a directory under leetcode/ with README.md, solution.py, test_solution.py, helpers.py, and playground.ipynb. lcpy gen creates them from JSON templates bundled with the package.
> Examples are backed by tests; copy them verbatim.

# Roman to Integer Python Solution with Tests

> Tested Python solution for LeetCode 13 with 16 pytest cases. Generate a practice environment with lcpy.

LeetCode 13, Easy. Topics: Hash Table, Math, String. [View on LeetCode](https://leetcode.com/problems/roman-to-integer/description/).

Generate this problem as a practice environment: tested reference solution, 16 [parametrized pytest cases](/practice/testing), and a playground notebook:

```bash theme={"theme":{"light":"github-light","dark":"github-dark"}}
lcpy gen -n 13   # by problem number
lcpy gen -s roman_to_integer   # by problem name
```

## Problem

Roman numerals are represented by seven different symbols: `I`, `V`, `X`, `L`, `C`, `D` and `M`.

**Symbol**       **Value**
I             1
V             5
X             10
L             50
C             100
D             500
M             1000

For example, `2` is written as `II` in Roman numeral, just two ones added together. `12` is written as `XII`, which is simply `X + II`. The number `27` is written as `XXVII`, which is `XX + V + II`.

Roman numerals are usually written largest to smallest from left to right. However, the numeral for four is not `IIII`. Instead, the number four is written as `IV`. Because the one is before the five we subtract it making four. The same principle applies to the number nine, which is written as `IX`. There are six instances where subtraction is used:

* `I` can be placed before `V` (5) and `X` (10) to make 4 and 9.
* `X` can be placed before `L` (50) and `C` (100) to make 40 and 90.
* `C` can be placed before `D` (500) and `M` (1000) to make 400 and 900.

Given a roman numeral, convert it to an integer.

### Examples

```
Input: s = "III"
Output: 3
Explanation: III = 3.
```

```
Input: s = "LVIII"
Output: 58
Explanation: L = 50, V= 5, III = 3.
```

```
Input: s = "MCMXCIV"
Output: 1994
Explanation: M = 1000, CM = 900, XC = 90 and IV = 4.
```

### Constraints

* 1 \<= s.length \<= 15
* `s` contains only the characters (`'I'`, `'V'`, `'X'`, `'L'`, `'C'`, `'D'`, `'M'`).
* It is guaranteed that `s` is a valid roman numeral in the range `[1, 3999]`.

## Solution

Reference implementation from [solution.py on GitHub](https://github.com/wislertt/leetcode-py/blob/main/leetcode/roman_to_integer/solution.py), full suite in [test\_solution.py](https://github.com/wislertt/leetcode-py/blob/main/leetcode/roman_to_integer/test_solution.py):

```python theme={"theme":{"light":"github-light","dark":"github-dark"}}
class Solution:
    # Time: O(n)
    # Space: O(1)
    def roman_to_int(self, s: str) -> int:
        values: dict[str, int] = {
            "I": 1,
            "V": 5,
            "X": 10,
            "L": 50,
            "C": 100,
            "D": 500,
            "M": 1000,
        }

        total = 0
        prev = 0
        # Walk right-to-left; subtract when a symbol is smaller than the previous one
        for char in reversed(s):
            current = values[char]
            if current < prev:
                total -= current
            else:
                total += current
            prev = current

        return total
```

## Complexity

| Time | Space |
| ---- | ----- |
| O(n) | O(1)  |

## Tags

[Grind](/catalog/grind), [NeetCode 250](/catalog/neetcode-250), [NeetCode All](/catalog/neetcode).