Approach: Scan, subtract when smaller precedes larger

def romanToInt(s):
    val = {'I':1,'V':5,'X':10,'L':50,'C':100,'D':500,'M':1000}
    total = 0
    for i in range(len(s)):
        if i+1 < len(s) and val[s[i]] < val[s[i+1]]:
            total -= val[s[i]]
        else:
            total += val[s[i]]
    return total

Complexity: O(n) time, O(1) space.

Full Solution — all languages

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

import java.util.*;
class Solution {
    public int romanToInt(String s) {
        Map<Character,Integer> val = new HashMap<>();
        val.put('I',1);
        val.put('V',5);
        val.put('X',10);
        val.put('L',50);
        val.put('C',100);
        val.put('D',500);
        val.put('M',1000);
        int total = 0;
        for (int i = 0; i < s.length(); i++) {
            if (i + 1 < s.length() && val.get(s.charAt(i)) < val.get(s.charAt(i + 1))) total -= val.get(s.charAt(i));
            else total += val.get(s.charAt(i));
        }
        return total;
    }
}

#include <bits/stdc++.h> using namespace std;
class Solution {
    public: int romanToInt(string s) {
        unordered_map<char,int> val = { {
            'I',1
        },
        {
            'V',5
        },
        {
            'X',10
        },
        {
            'L',50
        },
        {
            'C',100
        },
        {
            'D',500
        },
        {
            'M',1000
        }
        };
        int total = 0;
        for (int i = 0; i < (int)s.size(); i++) {
            if (i + 1 < (int)s.size() && val[s[i]] < val[s[i + 1]]) total -= val[s[i]];
            else total += val[s[i]];
        }
        return total;
    }
};

/**
 * @param {string} s
 * @return {number}
 */ var romanToInt = function(s) {
    const val = { I: 1, V: 5, X: 10, L: 50, C: 100, D: 500, M: 1000 };
    let total = 0;
    for (let i = 0; i < s.length; i++) {
        if (i + 1 < s.length && val[s[i]] < val[s[i + 1]]) total -= val[s[i]];
        else total += val[s[i]];
    }
    return total;
};

Roman to Integer

Example 1

Example 2

Example 3

Constraints