【leetcode】Roman to Integer

class Solution {
public:
    int romanToInt(string s) {
        // Note: The Solution object is instantiated only once and is reused by each test case.
        int n=s.size();
        if(n==0)
            return 0;
       
        map<char,int> m;
        m['I']=1;
        m['V']=5;
        m['X']=10;
        m['L']=50;
        m['C']=100;
        m['D']=500;
        m['M']=1000;
       
        int sum=m[s[n-1]];
        for(int i=n-2;i>=0;i--)
        {
            if(m[s[i]]<m[s[i+1]])
                sum-=m[s[i]];
            else
                sum+=m[s[i]];
        }
        return sum;
    }
};