Roman to Integer

From LeetCode

problem description / no solution provided

Solution in Python3

class Solution:
    def romanToInt(self, s):
        """
        :type s: str
        :rtype: int
        """
        sym_val = {
            "M" : 1000,
            "D" : 500,
            "C" : 100,
            "L" : 50,
            "X" : 10,
            "V" : 5,
            "I" : 1
        }
        int_ = 0
        max_val = 0
        for sym in s[::-1]:
            val = sym_val[sym]
            if val < max_val:
                int_ -= val
            else:
                int_ += val
                max_val = val
        return int_

I am lucky to get

Runtime: 120 ms, faster than 99.54% of Python3 online submissions for Roman to Integer.

Alternatively, aliasing max_val by prev_val, which means the value of previous symbol when we traverse s reversely. It just happened that prev_val == max_val because subtractions are restricted in roman numeral after all.

Variants

Left to right

Peek next symbol. sample1 sample2