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.

题解：

class Solution {
public:
  string intToRoman(int num) {
    string s;
    vector<int> m = {1, 4, 5, 9, 10, 40, 50, 90, 100, 400, 500, 900, 1000};
    vector<string> n = {"I", "IV", "V", "IX", "X", "XL", "L", "XC", "C", "CD", "D", "CM", "M"};
    for (int i = 12; i >= 0; i--) {
      int x = num / m[i];
      while (x > 0) {
        s += n[i];
        x--;
      }
      num %= m[i];
    }
    return s;
  }
};