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:

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:

(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)

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