## 13. Roman to Integer

Roman numerals are represented by seven different symbols: `I`

, `V`

, `X`

, `L`

, `C`

, `D`

and `M`

.

SymbolValueI 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 one's 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

**Example 2:**

Input:s = "IV"Output:4

**Example 3:**

Input:s = "IX"Output:9

**Example 4:**

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

**Example 5:**

Input:s = "MCMXCIV"Output:1994Explanation: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]`

.

## Rust Solution

```
struct Solution;
use std::collections::HashMap;
impl Solution {
fn roman_to_int(s: String) -> i32 {
let map: HashMap<char, i32> = vec![
('I', 1),
('V', 5),
('X', 10),
('L', 50),
('C', 100),
('D', 500),
('M', 1000),
]
.into_iter()
.collect();
let mut sum = 0;
let mut last = 0;
for c in s.chars() {
if let Some(&v) = map.get(&c) {
if v > last {
sum += v - last - last;
} else {
sum += v
}
last = v;
}
}
sum
}
}
#[test]
fn test() {
assert_eq!(Solution::roman_to_int(String::from("III")), 3);
assert_eq!(Solution::roman_to_int(String::from("IV")), 4);
assert_eq!(Solution::roman_to_int(String::from("IX")), 9);
assert_eq!(Solution::roman_to_int(String::from("LVIII")), 58);
assert_eq!(Solution::roman_to_int(String::from("MCMXCIV")), 1994);
}
```

Having problems with this solution? Click here to submit an issue on github.