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.

先前把900 400 90 40 9 4這些特殊情況另算，後來拜讀大神的部落格，才發現原來可以統一起來，程式碼量可以這麼短

class Solution {
public:
    string intToRoman(int num) {
    int a[]={1000,900,500,400,100,90,50,40,10,9,5,4,1};
    string b[]={"M","CM","D","CD","C","XC","L","XL","X","IX","V","IV","I"};
    int index=0;
    string result="";
    while(num>0){
        int temp=num/a[index];
        while(temp-->0)
            result+=b[index];
        num%=a[index++];
    }
    return result;
    }
};