## 13. 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 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: 58
Explanation: L = 50, V= 5, III = 3.
```

Example 5:

```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]`.

## 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);
}
``````

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