Leetcode #13 — Roman to Integer

3 min readJan 4, 2021

Hello everyone, today we’ll be looking at the Roman to Integer problem. This question is marked as “Easy” on Leetcode.

Problem Statement

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, 2 is written as II in Roman numeral, just two one's added together. 12 is written as XII, which is simply X + II. The number 27 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.

Examples from Leetcode

Example 1:

Input: s = "III"
Output: 3

Example 2:

Input: s = "IV"
Output: 4

Example 3:

Input: s = "IX"
Output: 9

Example 4:

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

Example 5:

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

Solution

Submission on Leetcode:

Explanation (with two examples):

1. Let's call: romanToInt('III')- Create a dictionary 'd' which holds the symbols and their corresponding values (refer to the problem description).- Replace the values "IV", "IX", "XL", "XC", "CD" and "CM" as needed.- Return the sum of the following (breaking down the return statement):for char in s, where s = 'III', get the corresponding value of the character from the dictionary 'd' with d[char], which in this case is d['I'] = 1 and since s = 'III' = 1 + 1 + 1 = 3- Return 3 as the result.
2. Let's call: romanToInt('IX')- Create a dictionary 'd' which holds the symbols and their corresponding values.- Replace "IX" with "VIIII" in s. As a result, s is now 'VIIII'- Return the sum of the following (breaking down the return statement):for char in s, where s = 'VIIII', get the corresponding value of the character from the dictionary 'd' with d[char], which in this case is d['V'] = 5 ... and so on. We would be summing up the values 5 + 1 + 1 + 1 + 1 = 9- Return 9 as the result.

That’s all, folks! Happy learning!

Dhairya Dave