12. Integer to Roman

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 an integer, convert it to a roman numeral.

Example 1:

```Input: num = 3
Output: "III"
```

Example 2:

```Input: num = 4
Output: "IV"
```

Example 3:

```Input: num = 9
Output: "IX"
```

Example 4:

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

Example 5:

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

Constraints:

• `1 <= num <= 3999`

12. Integer to Roman
``````struct Solution;

impl Solution {
fn int_to_roman(mut num: i32) -> String {
let mapping: Vec<(i32, &str)> = vec![
(1, "I"),
(2, "II"),
(3, "III"),
(4, "IV"),
(5, "V"),
(6, "VI"),
(7, "VII"),
(8, "VIII"),
(9, "IX"),
(10, "X"),
(20, "XX"),
(30, "XXX"),
(40, "XL"),
(50, "L"),
(60, "LX"),
(70, "LXX"),
(80, "LXXX"),
(90, "XC"),
(100, "C"),
(200, "CC"),
(300, "CCC"),
(400, "CD"),
(500, "D"),
(600, "DC"),
(700, "DCC"),
(800, "DCCC"),
(900, "CM"),
(1000, "M"),
(2000, "MM"),
(3000, "MMM"),
];
let mut res: String = "".to_string();
for (x, s) in mapping.iter().rev() {
if num >= *x {
res += s;
num -= x;
} else {
continue;
}
if num == 0 {
break;
}
}
res
}
}

#[test]
fn test() {
let num = 3;
let res = "III".to_string();
assert_eq!(Solution::int_to_roman(num), res);
let num = 1994;
let res = "MCMXCIV".to_string();
assert_eq!(Solution::int_to_roman(num), res);
}
``````