羅馬數字轉整數

其實只要弄懂原理，實現起來非常簡單：首先建立一個字典來映射符號和值，然後對字符串從左到右來，如果當前字符代表的值不小於其右邊，就加上該值；否則就減去該值。以此類推，最終得到的結果即是答案。

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]]:
                # print str(a[s[i]]) + " < " + str(a[s[i]])
                # print "ans = " + str(ans) + " - " +  str(a[s[i]])
                ans-=a[s[i]]
            else:
                # print str(a[s[i]]) + " >= " + str(a[s[i]])
                # print "ans = " + str(ans) + " + " +  str(a[s[i]])
                ans+=a[s[i]]
        return ans