just think of a transistion of decimal to roman,

using minus value to add a string at one time.

就想成十進位轉羅馬字的轉換過程，一次減掉一個level的數字來補上一個羅馬文字。

```
if num>3999 or num<1: #for 1~3999 guarentee
return ''
decimal = [1000,900,500,400,100,90,50,40,10,9,5,4,1]
roman = ['M','CM','D','CD','C','XC','L','XL','X','IX','V','IV','I'] #check by internet
ans = '' #initial
for i in range(len(decimal)): #main
while num >= decimal[i]:
num -= decimal[i]
ans += roman[i]
return ans
```