Leet Code 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, 2 is written as II in Roman numeral, just two ones added together. 12 is written as XII,       which is simply X + II. The number 27 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.

 

 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.

 

 Constraints:

• 1 <= s.length <= 15
• s contains only the characters ('I', 'V', 'X', 'L', 'C', 'D', 'M').
• It is guaranteed that s is a valid roman numeral in the range [1, 3999].

 Explanation:

The logic of the program is explained below with the example of MCMXCIV.

Take the value of each of the letters in the sequence:

1000 = M, 100 = C, 1000 = M, 10 = X, 100 = C, 1 = I, 5 = V

Keed adding the values in a variable. But for the second position onwards we

have check the difference of the value of the current position and the previous

position. If the difference is 4 or 9 or 40 or 90 or 400 or 900 then by the exception

rule we have to adjust the value.

For the example above, as 100-1000 = -900 ( i = 1 and i = 0) which is not under

exception rule we add 1000+100 = 1100

But for the next letter ('M') the value is 1000 ( i = 2 ) and 1000 - 100 = 900

( under exception rule), so instead of adding 1000 to 1100 we have to have add 900

to the value of the first letter ( since CM is forming a single value). For this

we need to subtract the value of the current letter (M = 1000) and the previous

letter ( C = 100 ) from the sum of 1000+100+1000 = 2100 and add 900.

Which gives 2100-1000 - 100 + 900 = 1900.

PYTHON CODE

class Solution(object):

    def romanToInt(self, s):

        temp = 0

        for i in range(len(s)):

            self.output += self.value[self.position(s[i])]

            #print ("BEFORE ADJUSTMENT",self.output)

            if i > 0:

                temp = (self.value[self.position(s[i])] -

                        self.value[self.position(s[i - 1])])

            if temp == 4 or temp == 9 or temp == 40 or temp == 90 or temp == 400 or temp == 900:

                self.output = self.output - self.value[self.position(

                    s[i - 1])] - self.value[self.position(s[i])] + temp

            #print ("AFTER ADJUSTMENT",self.output)

        return self.output

    def position(self, symb):

        counter = 0

        while symb != self.symbol[counter]:

            counter += 1

        return (counter)

    symbol = ['I', 'V', 'X', 'L', 'C', 'D', 'M']

    value = [1, 5, 10, 50, 100, 500, 1000]

    output = 0

obj = Solution()

s = str(input("Enter a valid roman numeral in range(1,3999):"))

print(obj.romanToInt(s))