Roman to Integer
<p>Roman numerals are represented by seven different symbols:&nbsp;<code>I</code>, <code>V</code>, <code>X</code>, <code>L</code>, <code>C</code>, <code>D</code> and <code>M</code>.</p>

<pre>
<strong>Symbol</strong>       <strong>Value</strong>
I             1
V             5
X             10
L             50
C             100
D             500
M             1000</pre>

<p>For example,&nbsp;<code>2</code> is written as <code>II</code>&nbsp;in Roman numeral, just two ones added together. <code>12</code> is written as&nbsp;<code>XII</code>, which is simply <code>X + II</code>. The number <code>27</code> is written as <code>XXVII</code>, which is <code>XX + V + II</code>.</p>

<p>Roman numerals are usually written largest to smallest from left to right. However, the numeral for four is not <code>IIII</code>. Instead, the number four is written as <code>IV</code>. Because the one is before the five we subtract it making four. The same principle applies to the number nine, which is written as <code>IX</code>. There are six instances where subtraction is used:</p>

<ul>
	<li><code>I</code> can be placed before <code>V</code> (5) and <code>X</code> (10) to make 4 and 9.&nbsp;</li>
	<li><code>X</code> can be placed before <code>L</code> (50) and <code>C</code> (100) to make 40 and 90.&nbsp;</li>
	<li><code>C</code> can be placed before <code>D</code> (500) and <code>M</code> (1000) to make 400 and 900.</li>
</ul>

<p>Given a roman numeral, convert it to an integer.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;III&quot;
<strong>Output:</strong> 3
<strong>Explanation:</strong> III = 3.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;LVIII&quot;
<strong>Output:</strong> 58
<strong>Explanation:</strong> L = 50, V= 5, III = 3.
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;MCMXCIV&quot;
<strong>Output:</strong> 1994
<strong>Explanation:</strong> M = 1000, CM = 900, XC = 90 and IV = 4.
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= s.length &lt;= 15</code></li>
	<li><code>s</code> contains only&nbsp;the characters <code>(&#39;I&#39;, &#39;V&#39;, &#39;X&#39;, &#39;L&#39;, &#39;C&#39;, &#39;D&#39;, &#39;M&#39;)</code>.</li>
	<li>It is <strong>guaranteed</strong>&nbsp;that <code>s</code> is a valid roman numeral in the range <code>[1, 3999]</code>.</li>
</ul>
Hash Table; Math; String
func romanToInt(s string) (ans int) {
	d := map[byte]int{'I': 1, 'V': 5, 'X': 10, 'L': 50, 'C': 100, 'D': 500, 'M': 1000}
	for i := 0; i < len(s)-1; i++ {
		if d[s[i]] < d[s[i+1]] {
			ans -= d[s[i]]
		} else {
			ans += d[s[i]]
		}
	}
	ans += d[s[len(s)-1]]
	return
}