Roman to Integer, thesarfo

Convert a Roman numeral string to an integer. (LeetCode problem)

Symbols: I=1, V=5, X=10, L=50, C=100, D=500, M=1000. Roman numerals are usually written largest-to-smallest left-to-right, but a smaller numeral before a larger one means subtract: IV=4 (5-1), IX=9 (10-1), XL=40 (50-10), XC=90 (100-10), CD=400 (500-100), CM=900 (1000-100).

Approach: map each symbol to its value, then traverse the string — if the current numeral is less than the next one, subtract its value; otherwise add it.

import java.util.HashMap;

public class RomanToInteger {
    public static int romanToInt(String s) {
        HashMap<Character, Integer> romanMap = new HashMap<>();
        romanMap.put('I', 1);
        romanMap.put('V', 5);
        romanMap.put('X', 10);
        romanMap.put('L', 50);
        romanMap.put('C', 100);
        romanMap.put('D', 500);
        romanMap.put('M', 1000);

        int total = 0;
        int length = s.length();

        for (int i = 0; i < length; i++) {
            int value = romanMap.get(s.charAt(i));

            // Check if the next numeral exists and is larger
            if (i + 1 < length && romanMap.get(s.charAt(i + 1)) > value) {
                total -= value; // Subtract if current is less than next
            } else {
                total += value; // Add otherwise
            }
        }

        return total;
    }
}

Handles all valid Roman numerals in the range [1, 3999]. Time: O(n). Space: O(1) — the map size is fixed regardless of input.