# 12.Integer to Roman <span class="tag" data-diff="medium" />
{%hackmd RN5D4nggQRO8wzNqxuvlNw %}
## 題目
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 an integer, convert it to a roman numeral. Input is guaranteed to be within the range from 1 to 3999.
**Example 1:**
```
Input: 3
Output: "III"
```
**Example 2:**
```
Input: 4
Output: "IV"
```
**Example 3:**
```
Input: 9
Output: "IX"
```
**Example 4:**
```
Input: 58
Output: "LVIII"
Explanation: L = 50, V = 5, III = 3.
```
**Example 5:**
```
Input: 1994
Output: "MCMXCIV"
Explanation: M = 1000, CM = 900, XC = 90 and IV = 4.
```
## 思路
羅馬數字可以看成兩兩一組，每一組可以涵蓋10進位制下的一個位數，所以我們可以把數字轉換成字串，從個位數開始parse，每一位數的規則都相同，故loop中可以抽象的用符號位置來表示。
```javascript
const symbol = "IVXLCDM".split("");
let ans = num.toString().split("").reverse().map((x,i) => {
let temp = symbol[2*i]+symbol[2*i]+symbol[2*i];
if (x == 9) return symbol[2*i] + symbol[2*i+2];
else if (x < 9 && x > 4) return symbol[2*i+1] + temp.substring(0,x-5);
else if (x == 4) return symbol[2*i] + symbol[2*i+1];
else return temp.substring(0,x);
});
return ans.reverse().join("");
```
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