Leetcode——罗马数字转换

代码如下：

class Solution(object):
    def romanToInt(self, s):
        """
        :type s: str
        :rtype: int
        """
        a = {
    
    'I':1, 'V':5, 'X':10, 'L':50, 'C':100, 'D':500, 'M':1000}        
        ans=0        
        for i in range(len(s)):            
            if i<len(s)-1 and a[s[i]]<a[s[i+1]]:                
                ans-=a[s[i]]
            else:
                ans+=a[s[i]]
        return ans