Posted on

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.
• `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. Input is guaranteed to be within the range from 1 to 3999.

Example 1:

```Input: "III"
Output: 3```

Example 2:

```Input: "IV"
Output: 4```

Example 3:

```Input: "IX"
Output: 9```

Example 4:

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

Example 5:

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

## Submission

My submission is like solving a regular expression problem.

```class Solution {
public int romanToInt(String s) {
Map<String, Integer> map = Map.of("I", 1, "V", 5, "X", 10, "L", 50, "C", 100, "D", 500, "M", 1000);
if(s.length() == 0) {
return 0;
}

if(s.length() == 1) {
return(map.get(s.substring(0, 1)));
}

String sub2 = s.substring(0, 2);

if(sub2.compareTo("IV") == 0) {
return 4 + romanToInt(s.substring(2));
}

if(sub2.compareTo("IX") == 0) {
return 9 + romanToInt(s.substring(2));
}

if(sub2.compareTo("XL") == 0) {
return 40 + romanToInt(s.substring(2));
}

if(sub2.compareTo("XC") == 0) {
return 90 + romanToInt(s.substring(2));
}

if(sub2.compareTo("CD") == 0) {
return 400 + romanToInt(s.substring(2));
}

if(sub2.compareTo("CM") == 0) {
return 900 + romanToInt(s.substring(2));
}

return map.get(s.substring(0, 1)) + romanToInt(s.substring(1));
}
}```