給定一個羅馬數字，將其轉換為整數。

Example 1:

Input: s = "III"
Output: 3
Explanation: III = 3.

Example 2:

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

Example 3:

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

解題：
觀察CM = 900, XC = 90 and IV = 4 當前字母 < 後面的字母 ， 則減去該數值 ，反之，若是大於後面的值，則加總該數。使用next要注意超出陣列的問題。

RomanValue = {
    "I" => 1,
    "V" => 5,
    "X" => 10,
    "L" => 50,
    "C" => 100,
    "D" => 500,
    "M" => 1000
}

def roman_to_int(s)
     (0..(s.length-1)).sum{ |index|
        if  RomanValue[s[index.next]].nil?
            RomanValue[s[index]] * 1
        else
          if RomanValue[s[index]] < RomanValue[s[index.next]]
            RomanValue[s[index]] * -1
          else
            RomanValue[s[index]] * 1
          end
        end
    }
end

如果要寫乾淨一點，可以使用fetch方法，加入預設值，減少判斷

def roman_to_int(s)
    (0..(s.length-1)).sum{ |index|
        if RomanValue[s[index]] < RomanValue.fetch(s[index.next],0)
            RomanValue[s[index]] * -1
        else
            RomanValue[s[index]] * 1
        end
    }
end