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, 2 is written as II in Roman numeral, just two one's added together. 12 is written as XII, which is simply X + II. The number 27 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.

Example 1:

Input: num = 3
Output: "III"

Example 2:

Input: num = 4
Output: "IV"

Example 3:

Input: num = 9
Output: "IX"

Example 4:

Input: num = 58
Output: "LVIII"
Explanation: L = 50, V = 5, III = 3.

Example 5:

Input: num = 1994
Output: "MCMXCIV"
Explanation: M = 1000, CM = 900, XC = 90 and IV = 4.

Constraints:

1 <= num <= 3999

实现思路：

数字转成罗马数字，本题的规则就算可以由{1,4,5,9，10}组成10以内的所有罗马数字，而本题最大要求组成的罗马数字不超过3999，其实是降低了难度，方法就是列出数组然后循环做减法输出即可。

AC代码：

class Solution {
public:
    string intToRoman(int num) {
        char str[15][5]={"I","IV","V","IX","X","XL","L","XC","C","CD","D","CM","M"};
        int i2s[15]={1,4,5,9,10,40,50,90,100,400,500,900,1000};
        string ans;
        int idx=12;
        while(idx>=0){
            if(num>=i2s[idx]){
                ans+=str[idx];
                num-=i2s[idx];
            }else idx--;
        }
        return ans;
    }
};