JavaScript Algorithms: Roman to Integer (LeetCode)

Anatolii Kurochkin
2 min readSep 30, 2020


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

For example, two is written as II in Roman numeral, just two one's added together. Twelve is written as, XIIwhich is simply X + II. The number twenty-seven is written as XXVII, which is XX + V + II.

Roman numerals are usually written largest to smallest from left to right. However, the numeral for four is not IIII. Instead, the number four is written as IV. Because the one is before the five we subtract it making four. The same principle applies to the number nine, which is written as IX. There are six instances where subtraction is used:

  • I can be placed before V (5) and X (10) to make 4 and 9.
  • X can be placed before L (50) and C (100) to make 40 and 90.
  • C can be placed before D (500) and M (1000) to make 400 and 900.

Given a roman numeral, convert it to an integer. Input is guaranteed to be within the range from 1 to 3999.

Example 1:

Input: "III"
Output: 3

Example 2:

Input: "IV"
Output: 4

Example 3:

Input: "IX"
Output: 9

Example 4:

Input: "LVIII"
Output: 58
Explanation: L = 50, V= 5, III = 3.

Example 5:

Input: "MCMXCIV"
Output: 1994
Explanation: M = 1000, CM = 900, XC = 90 and IV = 4.


First of all, need to create a Map with Roman-Integer key-value pairs. After that, we can loop through the string and count the result variable.

The problem with IX, IV, XC …

We need to figure out how we can count these Roman values.

The rule is pretty simple — if the next value is bigger than the current, we need to subtract the current value from the next. For example:

So. inside the for loop we’re going to create a current and next variable. And depending on whether the next is bigger than the current, we’re filling the result:

Thanks for reading! Looking forward to your feedback. See you soon ✌️