A roman to integer converter

For an interview question I made a roman to integer converter:

def romanToInt(self, s):
        """
        :type s: str
        :rtype: int
        """
        mapping = {"I": 1, "V":5, "X":10, "L":50, "C":100, "D":500, "M":1000}
        numeral_list = list(s)

        num_list = [[mapping[i], -1] for i in numeral_list]
        count = 0
        i = 0
        while(i < len(num_list) - 1):
            if num_list[i] < num_list[i + 1]:
                count += num_list[i + 1][0] - num_list[i][0]
                num_list[i+1][1] = 1
                num_list[i][1] = 1
                i += 2
            else:
                count += num_list[i][0]
                num_list[i][1] = 1
                i += 1
        if num_list[-1][1] == -1:
            count += num_list[-1][0]
        return count

As you can see I sometimes miss the last digit because I didn't want to get an index error. To avoid that I added an extra attribute that would check if the last element was checked or not (in cases where s[len(s)-2] s[len(s)-1] then s[len(s)-1] is not checked.

Having an extra check and using extra space just for one element is highly erroneous. Where am I going wrong in my logic?

You should stick to variable names according to the python style guide, PEP8. This means using lower_case for variables and functions.

You could also use more descriptive names, for example roman instead of s.

The complicated checking you do with having 2-tuples and deciding on that can be circumvented by iterating over zip(numbers, numbers[1:]) to have both the number and the following number available at the same time. Still, we need to manually add the last digit at the end. Similar to the answer to this question I would propose to use something like:

def roman_to_int(roman):
    """Convert from Roman numerals to an integer."""
    values = {'M': 1000, 'D': 500, 'C': 100, 'L': 50, 
              'X': 10, 'V': 5, 'I': 1}
    numbers = [values[char] for char in roman]
    total = 0
    for num1, num2 in zip(numbers, numbers[1:]):
        if num1 >= num2:
            total += num1
        else:
            total -= num1
    return total + num2

StackExchange Code Review Q#137915, answer score: 4