问题描述

Given a roman numeral, convert it to an integer.

Input is guaranteed to be within the range from 1 to 3999.

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补充说明：

这个题目的要求很简单，给定你一个罗马字符串，给出它的阿拉伯数字表示。

方案分析

1. 什么是罗马数字？
   
   维基百科说明 [1]: https://en.wikipedia.org/wiki/Roman_numerals "Roman numerals"
   
   简要的说明一下，

<table class="wikitable">
<tr>
<th>Symbol</th>
<td><span class="times-serif" title="Roman numeral">I</span></td>
<td><span class="times-serif" title="Roman numeral">V</span></td>
<td><span class="times-serif" title="Roman numeral">X</span></td>
<td><span class="times-serif" title="Roman numeral">L</span></td>
<td><span class="times-serif" title="Roman numeral">C</span></td>
<td><span class="times-serif" title="Roman numeral">D</span></td>
<td><span class="times-serif" title="Roman numeral">M</span></td>
</tr>
<tr>
<th>Value</th>
<td>1</td>
<td>5</td>
<td>10</td>
<td>50</td>
<td>100</td>
<td>500</td>
<td>1,000</td>
</tr>
</table>

上面表格给出的是常用的罗马字符对应的阿拉伯数值。

<strong>要注意的来了：</strong>


<table class="wikitable">
<tbody><tr>
<th>Number</th>
<td>4</td>
<td>9</td>
<td>40</td>
<td>90</td>
<td>400</td>
<td>900</td>
</tr>
<tr>
<th>Notation</th>
<td><span class="times-serif" title="Roman numeral">IV</span></td>
<td><span class="times-serif" title="Roman numeral">IX</span></td>
<td><span class="times-serif" title="Roman numeral">XL</span></td>
<td><span class="times-serif" title="Roman numeral">XC</span></td>
<td><span class="times-serif" title="Roman numeral">CD</span></td>
<td><span class="times-serif" title="Roman numeral">CM</span></td>
</tr>
</tbody></table>

• <span class="times-serif" title="Roman numeral">I</span> placed before <span class="times-serif" title="Roman numeral">V</span> or <span class="times-serif" title="Roman numeral">X</span> indicates one less, so four is <span class="times-serif" title="Roman numeral">IV</span> (one less than five) and nine is <span class="times-serif" title="Roman numeral">IX</span> (one less than ten)
• <span class="times-serif" title="Roman numeral">X</span> placed before <span class="times-serif" title="Roman numeral">L</span> or <span class="times-serif" title="Roman numeral">C</span> indicates ten less, so forty is <span class="times-serif" title="Roman numeral">XL</span> (ten less than fifty) and ninety is <span class="times-serif" title="Roman numeral">XC</span> (ten less than a hundred)
• <span class="times-serif" title="Roman numeral">C</span> placed before <span class="times-serif" title="Roman numeral">D</span> or <span class="times-serif" title="Roman numeral">M</span> indicates a hundred less, so four hundred is <span class="times-serif" title="Roman numeral">CD</span> (a hundred less than five hundred) and nine hundred is <span class="times-serif" title="Roman numeral">CM</span> (a hundred less than a thousand)<sup id="cite_ref-sun_5-0" class="reference">[5]
  
  简单翻译一下，就是：
  
  当I出现在V和X前面的时候，相当于减去1;
  
  当X出现在L和C前面的时候，相当与减去10；
  
  当C出现在D和M前面的时候，相当于减去100;

2. 前面那段是简介，基本要传递出来的就是两点：
   
   (1)哪个罗马字符对应哪个阿拉伯值。
   
   (2)一句话给你概括了，就是当后面的那个罗马数字比前面罗马数字大的话，相当于减去后面那个罗马数字的值。

python实现

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