# 13. Roman to Integer

[Link](https://leetcode.com/problems/roman-to-integer/description/)

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, two is written as `II` in Roman numeral, just two one's added together. Twelve is written as, `XII`, which is simply `X` + `II`. The number twenty seven 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.&#x20;
* `X` can be placed before `L` (50) and `C` (100) to make 40 and 90.&#x20;
* `C` can be placed before `D` (500) and `M` (1000) to make 400 and 900.

Given a roman numeral, convert it to an integer. Input is guaranteed to be within the range from 1 to 3999.

**Solution**

```
public static int romanToInt(String s) {
        int ret = 0;
        int len = s.length();
        for(int i = 0; i < len; i++){
            if(s.charAt(i)=='I'){
                ret += 1;
            }
            else if(s.charAt(i)=='X'){
                ret += 10;
                if(i >= 1 && s.charAt(i-1) == 'I'){
                    ret -= 2;
                }
            }
            else if(s.charAt(i)=='V'){
                ret += 5;
                if(i >= 1 && s.charAt(i-1) == 'I'){
                    ret -= 2;
                }
            }
            else if(s.charAt(i)=='L'){
                ret += 50;
                if(i >= 1 && s.charAt(i-1) == 'X'){
                    ret -= 20;
                }
            }
            else if(s.charAt(i)=='C'){
                ret += 100;
                if(i >= 1 && s.charAt(i-1) == 'X'){
                    ret -= 20;
                }
            }
            else if(s.charAt(i)=='D'){
                ret += 500;
                if(i >= 1 && s.charAt(i-1) == 'C'){
                    ret -= 200;
                }
            }
            else if(s.charAt(i)=='M'){
                ret += 1000;
                if(i >= 1 && s.charAt(i-1) == 'C'){
                    ret -= 200;
                }
            }
        }
        return ret;
    }
```
