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Convert Roman Numerals to Integers

Tyler Scott Williams on February 10, 2020

This post is part of my "LeetCode for 1x Developers" series, in which I struggle through LeetCode problems. Sometimes I figure it out, other times ...
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David Griffin • • Edited

Special casing all those checks is asking for problems though, and I'm pretty sure it's not that neat. If we assume that the input is a valid Roman numeral (which this code does do; "IIX" is not a valid Roman numeral, but your code will parse it as 10), then the rule is that "if a numeral is smaller than the next numeral, subtract its value". Using that rule gives a much neater function:

vals = {'I': 1, 'V': 5, 'X': 10, 'L': 50, 'C': 100, 'D': 500, 'M': 1000}

def roman_to_int(input_string: str) -> int:
    ints = [vals[x] for x in input_string] + [0]
    output = 0
    for current, next in zip(ints, ints[1:]):
        output += -current if current < next else current
    return output

(Apologies for using Python, but I don't trust my JS enough to write it error free) (Edited to add the vals dict so the code actually runs)

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Tyler Scott Williams •

Ah yeah! Good catch. I think that's a given in the LC problem but I forgot to say it. If it isn't, none of the LC cases provide invalid numerals, haha

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Philippe Ozil • • Edited

Interesting problem, thanks for sharing.

@everyone who might read and for the sake of not re-inventing the wheel, note that there's a CSS rule for list style:

ol { list-style-type: upper-roman; }