罗马数字是从左到右读取的，当您添加或减去每个符号的值时。
如果一个值低于下面的值，它将被减去。否则将被添加。
例如，我们要将罗马数字MCMLIV转换为阿拉伯数字：M = 1000 must be added, because the following letter C =100 is lower.
C = 100 must be subtracted because the following letter M =1000 is greater.
M = 1000 must be added, because the following letter L = 50 is lower.
L = 50 must be added, because the following letter I =1 is lower.
I = 1 must be subtracted, because the following letter V = 5 is greater.
V = 5 must be added, because there are no more symbols left.
我们现在可以计算出这个数字：1000 - 100 + 1000 + 50 - 1 + 5 = 1954def from_roman(num):
roman_numerals = {'I':1, 'V':5, 'X':10, 'L':50, 'C':100, 'D':500, 'M':1000}
result = 0
for i,c in enumerate(num):
if (i+1) == len(num) or roman_numerals[c] >= roman_numerals[num[i+1]]:
result += roman_numerals[c]
else:
result -= roman_numerals[c]
return result