RubyでLeetCodeを解いてみた Roman to Integer

Roman numerals are represented by seven different symbols: I, V, X, L, C, D and M.
Given a roman numeral, convert it to an integer. Input is guaranteed to be within the range from 1 to 3999.

# @param {String} s
# @return {Integer}
def roman_to_int(s)
    arr = s.split("")
    last_idx = arr.size - 1
    sum = 0
    arr.each_with_index do |str, idx|
        case str
        when "I"
            if idx != last_idx && (arr[idx + 1] == "V" || arr[idx + 1] == "X")
                sum -= 1
            else
                sum += 1
            end
        when "V"
            sum += 5
        when "X"
            if idx != last_idx && (arr[idx + 1] == "L" || arr[idx + 1] == "C")
                sum -= 10
            else
                sum += 10
            end
        when "L"
            sum += 50
        when "C"
            if idx != last_idx && (arr[idx + 1] == "D" || arr[idx + 1] == "M")
                sum -= 100
            else
                sum += 100
            end
        when "D"
            sum += 500
        when "M"
            sum += 1000
        end
    end
    sum
end

馬鹿正直に実装してみた😛