leetcode[13]Roman to Integer

Given a roman numeral, convert it to an integer.

Input is guaranteed to be within the range from 1 to 3999.

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