C++

class Solution {
public:
    int romanToInt(string s) {
        int n = s.length(), ans = 0;
        unordered_map<char, int> map;
        map['I'] = 1, map['V'] = 5;
        map['X'] = 10, map['L'] = 50;
        map['C'] = 100, map['D'] = 500;
        map['M'] = 1000;

        for (int i = 0; i < n; i++) {
            if (i < n - 1 && map[s[i]] < map[s[i + 1]]) 
                ans -= map[s[i]];
            else 
                ans += map[s[i]];
        }

        return ans;
    }
};

Python3

class Solution:
    def romanToInt(self, s: str) -> int:
        dic = {'I':1, 'V':5, 'X':10, 'L':50, 'C':100, 'D':500, 'M':1000}
        ans = 0
        n = len(s)
        for i in range(n):
            if i < n - 1 and dic[s[i]] < dic[s[i + 1]]:
                ans -= dic[s[i]]
            else:
                ans += dic[s[i]]
        return ans

Go

func romanToInt(s string) int {
    n := len(s)
    ans := 0
    myMap := map[byte]int {
        'I':1, 'V':5,
        'X':10, 'L':50,
        'C':100, 'D':500,
        'M':1000,
    }

    for i := 0; i < n; i++ {
        if i < n - 1 && myMap[s[i]] < myMap[s[i + 1]] {
            ans -= myMap[s[i]]
        } else {
            ans += myMap[s[i]]
        }
    }

    return ans
}