Roman to Integer

class Solution {
public:
    int romanToInt(string s) {
        int nLength=s.length();
        if(nLength<1)
            return 0;
        map<char,int> m_stoi;
        m_stoi[‘I‘]=1;
        m_stoi[‘V‘]=5;
        m_stoi[‘X‘]=10;
        m_stoi[‘L‘]=50;
        m_stoi[‘C‘]=100;
        m_stoi[‘D‘]=500;
        m_stoi[‘M‘]=1000;
        int i=nLength-1;
        int sum=m_stoi[s[i]];
　　　　 i--;
        while(i>=0)
        {
            if(m_stoi[s[i+1]]>m_stoi[s[i]])
                sum-=m_stoi[s[i]];
            else
                sum+=m_stoi[s[i]];
            i--;
        }
        return sum;
    }
};