LeetCode13. Roman to Integer

罗马数字转换整数

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本文介绍了一种将罗马数字转换为整数的方法。通过解析罗马数字的构成规则，特别是特殊情况如IV代表4的情况，实现了一个简单的算法。该算法逐字符读取罗马数字，并根据前后字符的关系决定加减操作。

Roman numerals are represented by seven different symbols: I, V, X, L, C, D and M.

Symbol       Value
I             1
V             5
X             10
L             50
C             100
D             500
M             1000

For example, two is written as II in Roman numeral, just two one's added together. Twelve is written as, XII, which is simply X + II. The number twenty seven is written as XXVII, which is XX + V + II.

Roman numerals are usually written largest to smallest from left to right. However, the numeral for four is not IIII. Instead, the number four is written as IV. Because the one is before the five we subtract it making four. The same principle applies to the number nine, which is written as IX. There are six instances where subtraction is used:

I can be placed before V (5) and X (10) to make 4 and 9.
X can be placed before L (50) and C (100) to make 40 and 90.
C can be placed before D (500) and M (1000) to make 400 and 900.

Given a roman numeral, convert it to an integer. Input is guaranteed to be within the range from 1 to 3999.

Example 1:

Input: "III"
Output: 3

Example 2:

Input: "IV"
Output: 4

Example 3:

Input: "IX"
Output: 9

Example 4:

Input: "LVIII"
Output: 58
Explanation: C = 100, L = 50, XXX = 30 and III = 3.

Example 5:

Input: "MCMXCIV"
Output: 1994
Explanation: M = 1000, CM = 900, XC = 90 and IV = 4.

<思路>看着比较麻烦，之前也一直没做。看了discuss，发现只要是小的在左面，就是减去；只要是小的在右面，就是加上。并不需要整体去看这个罗马数字，一个字母一个字母看就行。

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