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：

　　整形转罗马数字时，1,2,..,9，10,20,...,90，100,200,...,900，...的转换方式是相同的，不同的只是对应的字母。因此将一个整数转换为罗马数字只需将整数分开成个位、十位...单独转换，最后在整合在一起即可。

代码：

    //Integer to Roman
    public static String intToRoman(int num){
       
        List<String> l = new ArrayList<>();
        int flag = 0;
        String a = "X",b = "V",c = "I";
        
        while (num!=0) {
            int n = num%10;
            num = num/10;
            if(flag == 1){
                a = "C";b="L";c="X";
            }
            if(flag == 2){
                a = "M";b="D";c="C";
            }
            if(flag == 3){
                c = "M";
            }
            String str = romanSub(n,a,b,c);
            l.add(str);
            flag ++;
        }
        
        String result = "";
        for (int i = l.size()-1; i >= 0; i--) {
            result+=l.get(i);
        }
        
        return result;
    }
    
    //a 10  b 5  c 1 
    public static String romanSub(int num,String a,String b,String c){
        String res = "";
        if(num<5){
            if(num<4){
                for (int i = 0; i < num; i++) {
                    res += c;
                }
            }else{
                res = c+b;
            }
        }else if(num == 5){
            res = b;
        }else{
            if(num<9){
                res = b;
                for (int i = 5; i < num; i++) {
                    res += c;
                }
            }else{
                res = c+a;
            }
        }
        return res;
    }