013. Roman to Integer

问题

Given a roman numeral, convert it to an integer.

Input is guaranteed to be within the range from 1 to 3999.

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思路

首先要知道罗马数字的规律：

然后还有一个规则是，罗马数字从左自右相加，但是如果小数字A在大数字B之前，表示B-A

VI = 5+1 = 6
IV = 5-1 = 4

因此，利用2个变量保存当前数字和之前的数字就行了。因为这题很简单，用python比较方便，我就用python做的

class Solution(object):
    def romanToInt(self, s):
        sum=0
        pre = 2000
        cur = 0
        Map = {'I':1,'V':5,'X':10,'L':50,'C':100,'D':500,'M':1000}
        for i in range(len(s)):
            cur = Map[s[i]]
            sum = sum+Map[s[i]]
            if cur > pre :
                sum = sum-2*pre
            pre = cur
        return sum