leetcode.0013. Roman to Integer

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本文详细解析了如何将罗马数字转换为整数，并提供了一个Python实现的例子。介绍了罗马数字的基本构成及其特殊减法规则，适用于1到3999范围内的罗马数字。

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leetcode.0013. Roman to Integer

题目

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 4:

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

作者答案（python3）

class Solution:
    def romanToInt(self, s):
        """
        :type s: str
        :rtype: int
        """

        # dictionary of Roman numerals 
        dictRoman = {'I':1, 'V':5, 'X':10, 'L':50, 'C':100, 'D':500, 'M':1000} 
        num = 0
        s_len = len(s)
        for i in range(s_len):
            if s_len-i-1>0: 
                if s[i:i+2] in {'IV','IX','XL','XC','CD','CM'}:# set of subtraction
                    num -= dictRoman[s[i]]
                else:
                    num += dictRoman[s[i]]
            else:
                num += dictRoman[s[i]]

        return num