ARTS-Week-04

每周完成一个ARTS：每周至少做一个 leetcode 的算法题、阅读并点评至少一篇英文技术文章、学习至少一个技术技巧、分享一篇有观点和思考的技术文章。（也就是 Algorithm、Review、Tip、Share 简称ARTS）

1.Algorithm

Roman to Integer

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:

1 2	Input: "III" Output: 3

Example 2:

1 2	Input: "IV" Output: 4

Example 3:

1 2	Input: "IX" Output: 9

Example 4:

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2
3

Input: "LVIII"
Output: 58
Explanation: L = 50, V= 5, III = 3.

Example 5:

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2
3

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

Code:

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