Roman to Integer

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.   **Example 1:** **Input:** s = "III" **Output:** 3 **Explanation:** III = 3. ``` **Example 2:** **Input:** s = "LVIII" **Output:** 58 **Explanation:** L = 50, V= 5, III = 3. ``` **Example 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]`.