题目描述

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.

分析

建立映射表，把特殊情况也加入到映射表中，包括4, 9, 40等等，不需要分类考虑之类的，这样更简便。
注意：虽然映射表的键是从大到小初始化，但是由于是有序的map，还是会重排为从小到大，还不如一开始就从小到大初始化。但是又不能用unordered_map，因为它是完全无序的。

class Solution {
public:
    string intToRoman(int num) {
        map<int,string> m{{1000,"M"},{900,"CM"},{500,"D"},
        {400,"CD"},{100,"C"},{90,"XC"},{50,"L"},{40,"XL"},{10,"X"},
        {9,"IX"},{5,"V"},{4,"IV"},{1,"I"}};
        string ans;
        for(auto it=m.rbegin();it!=m.rend();it++){
            while(num>=it->first){
                ans+=it->second;
                num-=it->first;
            }
        }
        return ans;
    }
};