12. 整数转罗马数字

题目：
罗马数字包含以下七种字符： I， V， X， L，C，D 和 M。

字符 数值
I 1
V 5
X 10
L 50
C 100
D 500
M 1000

代码：

class Solution {
public String intToRoman(int num) {
if(num<1||num>3999){return "";}
int[] values={1000,900,500,400,100,90,50,40,10,9,5,4,1};
String[] ch={"M","CM","D","CD","C","XC","L","XL","X","IX","V","IV","I"};
StringBuilder str=new StringBuilder();
for(int i=0;i<values.length;i++){
while(num>=values[i]){
str.append(ch[i]);
num-=values[i];
}
}
return str.toString();

}

}