13. Roman to Integer.py

'''
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.
'''

class Solution:
    def romanToInt(self, s: str) -> int:
        symbols = {'I':1,'V':5,'X':10,'L':50,'C':100,'D':500,'M':1000}
        res = 0
        current = 0
        prev = 0
        for c in s[::-1]:
            current = symbols[c]
            if current>=prev:
                res += current
            else:
                res -= current
            prev = current
        return res