13 Roman to Integer(罗马数字转int Easy)

题目意思：罗马数字转int

思路：字符串从最后一位开始读，IV：+5-1

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