LeetCode 13. Roman to Integer

Solution1：貌似是我的答案，还是参考的谁的？？
和12题相反的一个题

class Solution {
public:
    int romanToInt(string s) {
        if (s.length()==0) return 0;  
        unordered_map<char, int> m ={{'I', 1}, 
                           {'V', 5}, 
                           {'X', 10}, 
                           {'L', 50}, 
                           {'C', 100}, 
                           {'D', 500}, 
                           {'M', 1000}
                          };  
        int n = s.length();  
        int result = m[s[n-1]];  
        for (int i=n-2; i>=0; i--) {  
            if (m[s[i]] >= m[s[i+1]])  
                result += m[s[i]];  
            else 
                result -= m[s[i]];  
        }  
        return result;  
    }
};