Second Largest Digit in a String
<p>Given an alphanumeric string <code>s</code>, return <em>the <strong>second largest</strong> numerical digit that appears in </em><code>s</code><em>, or </em><code>-1</code><em> if it does not exist</em>.</p>

<p>An <strong>alphanumeric</strong><strong> </strong>string is a string consisting of lowercase English letters and digits.</p>

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

<pre>
<strong>Input:</strong> s = &quot;dfa12321afd&quot;
<strong>Output:</strong> 2
<strong>Explanation:</strong> The digits that appear in s are [1, 2, 3]. The second largest digit is 2.
</pre>

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

<pre>
<strong>Input:</strong> s = &quot;abc1111&quot;
<strong>Output:</strong> -1
<strong>Explanation:</strong> The digits that appear in s are [1]. There is no second largest digit. 
</pre>

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

<ul>
	<li><code>1 &lt;= s.length &lt;= 500</code></li>
	<li><code>s</code> consists of only lowercase English letters and digits.</li>
</ul>
Hash Table; String
func secondHighest(s string) int {
	a, b := -1, -1
	for _, c := range s {
		if c >= '0' && c <= '9' {
			v := int(c - '0')
			if v > a {
				b, a = a, v
			} else if v > b && v < a {
				b = v
			}
		}
	}
	return b
}