# 133. roman numerals to integer

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easy - accepted / - tried

Roman numerals are represented by combinations of following seven symbols, each with a fixed integer value.

Symbol | I | V | X | L | C | D | M |
---|---|---|---|---|---|---|---|

Value | 1 | 5 | 10 | 50 | 100 | 500 | 1000 |

For Standard form, subtractive notation is used, meaning 4 is `IV`

rather than `IIII`

, 9 is `IX`

rather than `VIIII`

. Same rule applies to 40(`XL`

) and 900(`CM`

) .etc.

Simply speaking, the roman numerals in standard form follow these rules.

- symbols are listed from highest to lowest, from left to right
- from left to right, if the next symbol value is bigger than current one, it means subtracting, otherwise adding.

Please implement `romanToInteger()`

. The input are all valid strings.

```
romanToInteger('CXXIII')
// 123
romanToInteger('MCMXCIX')
// 1999
romanToInteger('MMMCDXX')
// 3420
```