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
impl Solution {
    pub fn second_highest(s: String) -> i32 {
        let mut first = -1;
        let mut second = -1;
        for c in s.as_bytes() {
            if char::is_digit(*c as char, 10) {
                let num = (c - b'0') as i32;
                if first < num {
                    second = first;
                    first = num;
                } else if num < first && second < num {
                    second = num;
                }
            }
        }
        second
    }
}
