13_Roman to Integer

class Solution {
public:
    int romanToInt(string s) {
        int sum = 0;
        string pre_str;
        
        for(int i = 0; i < s.size(); i++){
            pre_str = s.substr(i,1);
            if (pre_str == "I"){
                
                if(s.substr(i+1,1) == "V" ||s.substr(i+1,1) == "X" )
                    sum = sum - 1;
                else
                    sum = sum + 1;
            }
            else if (pre_str == "V")
                sum = sum + 5;
            else if (pre_str == "X"){
                if(s.substr(i+1,1) == "L" ||s.substr(i+1,1) == "C" )
                    sum = sum - 10;
                else
                    sum = sum + 10;
            }
            else if (pre_str == "L")
                sum = sum + 50;
            else if (pre_str == "C"){
                if(s.substr(i+1,1) == "D" ||s.substr(i+1,1) == "M" )
                    sum = sum - 100;
                else
                    sum = sum + 100;
            }
            else if (pre_str == "D")
                sum = sum + 500;
            else if (pre_str == "M")
                sum = sum + 1000;
            else
                sum = 0;
            
        }
               return sum;
            
    }
};