Excel Sheet Column Number
<p>Given a string <code>columnTitle</code> that represents the column title as appears in an Excel sheet, return <em>its corresponding column number</em>.</p>

<p>For example:</p>

<pre>
A -&gt; 1
B -&gt; 2
C -&gt; 3
...
Z -&gt; 26
AA -&gt; 27
AB -&gt; 28 
...
</pre>

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

<pre>
<strong>Input:</strong> columnTitle = &quot;A&quot;
<strong>Output:</strong> 1
</pre>

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

<pre>
<strong>Input:</strong> columnTitle = &quot;AB&quot;
<strong>Output:</strong> 28
</pre>

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

<pre>
<strong>Input:</strong> columnTitle = &quot;ZY&quot;
<strong>Output:</strong> 701
</pre>

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

<ul>
	<li><code>1 &lt;= columnTitle.length &lt;= 7</code></li>
	<li><code>columnTitle</code> consists only of uppercase English letters.</li>
	<li><code>columnTitle</code> is in the range <code>[&quot;A&quot;, &quot;FXSHRXW&quot;]</code>.</li>
</ul>
Math; String
func titleToNumber(columnTitle string) (ans int) {
	for _, c := range columnTitle {
		ans = ans*26 + int(c-'A'+1)
	}
	return
}