<p>A message containing letters from <code>A-Z</code> can be <strong>encoded</strong> into numbers using the following mapping:</p>
<pre>
'A' ->"1"
'B' ->"2"
...
'Z' ->"26"
</pre>
<p>To <strong>decode</strong> an encoded message, all the digits must be grouped then mapped back into letters using the reverse of the mapping above (there may be multiple ways). For example, <code>"11106"</code> can be mapped into:</p>
<ul>
<li><code>"AAJF"</code> with the grouping <code>(1 1 10 6)</code></li>
<li><code>"KJF"</code> with the grouping <code>(11 10 6)</code></li>
</ul>
<p>Note that the grouping <code>(1 11 06)</code> is invalid because <code>"06"</code> cannot be mapped into <code>'F'</code> since <code>"6"</code> is different from <code>"06"</code>.</p>
<p>Given a string <code>s</code> containing only digits, return <em>the <strong>number</strong> of ways to <strong>decode</strong> it</em>.</p>
<p>The test cases are generated so that the answer fits in a <strong>32-bit</strong> integer.</p>
<strong>Explanation:</strong>"06" cannot be mapped to "F" because of the leading zero ("6" is different from "06").
</pre>
<p> </p>
<p><strong>Constraints:</strong></p>
<ul>
<li><code>1 <= s.length <= 100</code></li>
<li><code>s</code> contains only digits and may contain leading zero(s).</li>
