【LeetCode】13. Roman to Integer Python3解法

题目描述：

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.

例子：

Example 1:

Input: "III"
Output: 3

Example 2:

Input: "IV"
Output: 4

Example 3:

Input: "IX"
Output: 9

思路：

可以发现一个规律就是只有当后边一位大于前边的一位时，减去前边的一位的数即可。

代码：

class Solution:
    def romanToInt(self, s: str) -> int:
        d = {"I":1,"V":5,"X":10,"L":50,"C":100,"D":500,"M":1000}
        res = [d[j] for j in s]
        for i in range(len(res)-1):
            if res[i+1] > res[i]:
                res[i] = -res[i]
        return sum(res)

注解：

[d[j] for j in s]意思是将输入的string中的字符分别在字典d中找到相应的value并存放到list中。这样方便之后sum函数的计算。