Roman to Integer
(#13), Easy
Category: String, Hash Table, Math
Problem Statement
Given a Roman numeral, convert it to an integer. Roman numerals are represented by seven different symbols: I, V, X, L, C, D and M. Symbol | Value -------|------ I | 1 V | 5 X | 10 L | 50 C | 100 D | 500 M | 1000 Roman numerals are usually written largest to smallest from left to right. However, in a few special cases, we subtract values — such as IV = 4 or IX = 9. You are given a string representing a Roman numeral. Convert it to the corresponding integer.
Examples
Input: s = "III"
Output: 3
Explanation: "III" = 1 + 1 + 1 = 3
Input: s = "LVIII"
Output: 58
Explanation: "L" = 50, "V" = 5, "III" = 3 → Total = 58
Input: s = "MCMXCIV"
Output: 1994
Explanation: "M" = 1000, "CM" = 900, "XC" = 90, "IV" = 4 → Total = 1994
Approach
Map-Based Iteration with Subtractive Pair Check
- Create a map with values for single Roman symbols and subtractive pairs.
- Initialize `intVal = 0` and start iterating over the string with index `i`.
- Check if `s[i] + s[i+1]` forms a valid subtractive pair present in the map.
- - If yes: add its value and move `i` forward by 2.
- - Else: add value of single character `s[i]` and increment `i` by 1.
- Repeat until end of string and return the final `intVal`.
// 🔹 JavaScript Implementation
var romanToInt = function (s) {
const map = new Map([
["I", 1], ["V", 5], ["X", 10], ["L", 50],
["C", 100], ["D", 500], ["M", 1000],
["IV", 4], ["IX", 9], ["XL", 40], ["XC", 90],
["CD", 400], ["CM", 900]
]);
let intVal = 0;
for (let i = 0; i < s.length; ) {
if (i + 1 < s.length && map.has(s[i] + s[i + 1])) {
intVal += map.get(s[i] + s[i + 1]);
i += 2;
} else {
intVal += map.get(s[i]);
i++;
}
}
return intVal;
};
console.log(romanToInt("III")); // 3
console.log(romanToInt("LVIII")); // 58
console.log(romanToInt("MCMXCIV")); // 1994Complexity
Time: O(n)
Space: O(1)