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 an integer, convert it to a roman numeral.

**Example 1:**

Input:num = 3Output:"III"

**Example 2:**

Input:num = 4Output:"IV"

**Example 3:**

Input:num = 9Output:"IX"

**Example 4:**

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

**Example 5:**

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

**Constraints:**

`1 <= num <= 3999`

```
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);
}
```