<p>Given a string <code>s</code>, determine if it is <strong>valid</strong>.</p>
<p>A string <code>s</code> is <strong>valid</strong> if, starting with an empty string <code>t = ""</code>, you can <strong>transform </strong><code>t</code><strong> into </strong><code>s</code> after performing the following operation <strong>any number of times</strong>:</p>
<ul>
<li>Insert string <code>"abc"</code> into any position in <code>t</code>. More formally, <code>t</code> becomes <code>t<sub>left</sub> + "abc" + t<sub>right</sub></code>, where <code>t == t<sub>left</sub> + t<sub>right</sub></code>. Note that <code>t<sub>left</sub></code> and <code>t<sub>right</sub></code> may be <strong>empty</strong>.</li>
</ul>
<p>Return <code>true</code><em>if </em><code>s</code><em> is a <strong>valid</strong> string, otherwise, return</em><code>false</code>.</p>
