Leetcode: Roman to Integer

I attended a interview prep practice today and the question at hand was to convert a Roman numeral to an integer.

Roman to Integer - LeetCode

The question and examples are:

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, XII, which 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.

My partner interviewee, Brian, and I came up with a solution.

/**
 * @param {string} s
 * @return {number}
 */
var romanToInt = function(s) {
    const numHash = {
        "I" : 1,
        "V" : 5,
        "X" : 10,
        "L" : 50,
        "C" : 100,
        "D" : 500,
        "M" : 1000
    }
let num = [];
    let arr = s.split("");
    for(let i = 0; i < arr.length; i++){
        if (numHash[arr[i]] < numHash[arr[i + 1]]){
           num.push(numHash[arr[i+1]] - numHash[arr[i]])
           i++
        } else {
            num.push(numHash[arr[i]]);
        }    
    }
    console.log(num)
    return num.reduce((accumulator, currentVal) => accumulator + currentVal);
};

We made a hash map to convert all the single letters to numbers. Then, we iterate through the string as an array. If the current numeral is less than the next one, then we know we need to subtract the two. Else, we just push the current number into a num array where we store all the separate values.

Lastly, we use reduce to add up all of the values in num.

It was a little confusing at first, but my partner was able to come up with some great ideas. I’d definitely need more practice in this area!

Stay safe and social distance!