Roman to Integer

题目链接

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: L = 50, V= 5, III = 3.

Example 5:

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

解题思路：

首先要明白罗马数字的规则,详解百度百科
其次，本题中所有输入均为罗马数字，所以不需要验证，只需要做转换即可。
思路:
首先取出罗马数字的第一个字符所对应的值，然后每次跟前面的数字比较，如果小于等于前面的数字，我们先加上当前的数字，如果大于的前面的数字，我们加上当前的数字减去二倍前面的数字，这样可以把在上一个循环多加数减掉。

python3代码实现：

class Solution:
    def romanToInt(self, s):

        """
        :type s: str
        :rtype: int
        """

        length = len(s)
        d = {'I': 1, 'V': 5, 'X': 10, 'L': 50, 'C': 100, 'D': 500, 'M': 1000}
        val = 0
        if length == 1:
            return d[s[0]]
        val += d[s[0]]
        for i in range(1, len(s)):
            if d[s[i]] <= d[s[i - 1]]:
                val += d[s[i]]
            else:
                val += d[s[i]]
                val -= 2 * d[s[i-1]]
        return val