Lintcode 419. Roman to Integer (Medium) (Python)

Roman to Integer

Description:

Given a roman numeral, convert it to an integer.

The answer is guaranteed to be within the range from 1 to 3999.

Example
IV -> 4

XII -> 12

XXI -> 21

XCIX -> 99

Clarification
What is Roman Numeral?

https://en.wikipedia.org/wiki/Roman_numerals
https://zh.wikipedia.org/wiki/罗马数字
http://baike.baidu.com/view/42061.htm

Code:

class Solution:
    """
    @param s: Roman representation
    @return: an integer
    """
    def romanToInt(self, s):
        # write your code here
        digits = {"I":1, "V":5, "X":10, "L":50, "C":100, "D":500, "M":1000}
        res = 0
        pre = 1
        for i in range(len(s)-1, -1, -1):
            if digits[s[i]]>=pre:
                pre = digits[s[i]]
                res += digits[s[i]]
            else:
                res -= digits[s[i]]

        return res