13. Roman to Integer

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