13. Roman to Integer

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.

# @param {String} s
# @return {Integer}

def roman_to_int(s)
  
  total = 0
  current = 0
  new_arr = s.chars
  new_len = new_arr.length
  
  begin
    case new_arr[current]
      when "I"
        if new_arr[current+1] == "V"
          total += 4
          current += 2
        elsif new_arr[current+1] == "X"
          total += 9
          current += 2
        else
          total += 1
          current += 1
        end
      when "V"
        total += 5
        current += 1
      when "X"
        if new_arr[current+1] == "L"
          total += 40
          current += 2
        elsif new_arr[current+1] == "C"
          total += 90
          current += 2
        else
          total += 10
          current += 1
        end
      when "L"
        total += 50
        current += 1
      when "C"
        if new_arr[current+1] == "D"
          total += 400
          current += 2
        elsif new_arr[current+1] == "M"
          total += 900
          current += 2
        else
          total += 100
          current += 1
        end
      when "D"
        total += 500
        current += 1
      when "M"
        total += 1000
        current += 1
    end
  end while current < new_len
  
  total
  
end