Roman to Integer

Letter Combinations of a Phone Number

Palindrome Linked List

Convert a valid canonical Roman numeral into its integer value.

Convert a valid canonical Roman numeral to an integer. A smaller symbol before a larger symbol is subtractive; otherwise values are added.

**Input / output**
- Input: `s: string`
- Output: integer value

**Examples**
1. `"III"` returns `3`.
2. `"MCMXCVI"` returns `1996`.

**Constraints**
- `1 <= s.length <= 15`
- `s` is a valid uppercase canonical Roman numeral representing `1..3999`.

**Follow-up**
Can you express the conversion as a single left-to-right pass without special-casing each subtractive pair?

- You are given a Roman numeral, and you need to convert it to an integer.
- Roman numerals are made up of symbols (`I`, `V`, `X`, `L`, `C`, `D`, `M`) with respective values.
- The numeral can range from 1 to 3999, and subtraction cases (like `IV` for 4) must be handled.

Understanding the Problem

- Roman numerals are represented by seven different symbols with specific values:
```
Symbol    Value 
 I         1
 V         5
 X         10
 L         50
 C         100
 D         500
 M         1000
```
- Subtraction cases include:
  - I before V (5) and X (10) to make 4 and 9.
  - X before L (50) and C (100) to make 40 and 90.
  - C before D (500) and M (1000) to make 400 and 900.

List Roman Numerals and Values

- Use a dictionary to map Roman numeral symbols to their corresponding integer values.
- This will allow for constant-time lookups during conversion.

Create a Mapping for Roman Numerals

- To process the Roman numeral, iterate through the string from left to right.
- For each character, look up its corresponding value using the dictionary.

Iterate Through the String

- Roman numerals use subtraction in specific cases (like `IV` = 4).
- If a smaller value comes before a larger one, subtract the smaller value instead of adding it.

Handle Subtraction Cases

- After processing all characters in the Roman numeral string, return the accumulated total value, which represents the integer conversion.

Return the Final Sum

- Make sure that input Roman numerals are valid and within the range (1-3999).
- Since the problem guarantees this, we don't need extra validation, but this could be added in a more general implementation.