Solution for Roman to Integer
05 Aug 2018- 倒序遍历求值
罗马数字存在以下特征：
| key | value |
|---|---|
| I | 1 |
| IV | 4 |
| V | 5 |
| IX | 9 |
| X | 10 |
| XL | 40 |
| L | 50 |
| XC | 90 |
| C | 100 |
| CD | 400 |
| D | 500 |
| CM | 900 |
| M | 1000 |
因此可以通过倒序遍历，判断当前字母和前一个字母的大小，求和时对应+/-当前字母代表的数值，最终累加得到结果：
class Solution(object):
def romanToInt(self, s):
"""
:type s: str
:rtype: int
"""
numMap = {'I':1, 'V':5, 'X':10, 'L':50, 'C':100, 'D':500, 'M':1000}
total = 0
prev = 0
for char in s[::-1]:
curr = numMap[char]
if curr < prev:
total -= curr
else:
total += curr
prev = curr
return total