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
class Solution {
public:
    int secondHighest(string s) {
        int a = -1, b = -1;
        for (char& c : s) {
            if (isdigit(c)) {
                int v = c - '0';
                if (v > a) {
                    b = a, a = v;
                } else if (v > b && v < a) {
                    b = v;
                }
            }
        }
        return b;
    }
};