leetcode-problemset/leetcode/problem/decode-ways.html

<p>A message containing letters from <code>A-Z</code> can be <strong>encoded</strong> into numbers using the following mapping:</p>

<pre>
&#39;A&#39; -&gt; &quot;1&quot;
&#39;B&#39; -&gt; &quot;2&quot;
...
&#39;Z&#39; -&gt; &quot;26&quot;
</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>&quot;11106&quot;</code> can be mapped into:</p>

<ul>
	<li><code>&quot;AAJF&quot;</code> with the grouping <code>(1 1 10 6)</code></li>
	<li><code>&quot;KJF&quot;</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>&quot;06&quot;</code> cannot be mapped into <code>&#39;F&#39;</code> since <code>&quot;6&quot;</code> is different from <code>&quot;06&quot;</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>

<p>&nbsp;</p>
<p><strong>Example 1:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;12&quot;
<strong>Output:</strong> 2
<strong>Explanation:</strong> &quot;12&quot; could be decoded as &quot;AB&quot; (1 2) or &quot;L&quot; (12).
</pre>

<p><strong>Example 2:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;226&quot;
<strong>Output:</strong> 3
<strong>Explanation:</strong> &quot;226&quot; could be decoded as &quot;BZ&quot; (2 26), &quot;VF&quot; (22 6), or &quot;BBF&quot; (2 2 6).
</pre>

<p><strong>Example 3:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;06&quot;
<strong>Output:</strong> 0
<strong>Explanation:</strong> &quot;06&quot; cannot be mapped to &quot;F&quot; because of the leading zero (&quot;6&quot; is different from &quot;06&quot;).
</pre>

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

<ul>
	<li><code>1 &lt;= s.length &lt;= 100</code></li>
	<li><code>s</code> contains only digits and may contain leading zero(s).</li>
</ul>