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leetcode-cn/problem (English)/解码方法(English) [decode-ways].html
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<p>A message containing letters from <code>A-Z</code> can be <strong>encoded</strong> into numbers using the following mapping:</p>
<p>You have intercepted a secret message encoded as a string of numbers. The message is <strong>decoded</strong> via 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><code>&quot;1&quot; -&gt; &#39;A&#39;<br />
&quot;2&quot; -&gt; &#39;B&#39;<br />
...<br />
&quot;25&quot; -&gt; &#39;Y&#39;<br />
&quot;26&quot; -&gt; &#39;Z&#39;</code></p>
<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>
<p>However, while decoding the message, you realize that there are many different ways you can decode the message because some codes are contained in other codes (<code>&quot;2&quot;</code> and <code>&quot;5&quot;</code> vs <code>&quot;25&quot;</code>).</p>
<p>For example, <code>&quot;11106&quot;</code> can be decoded 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>
<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>
<li>The grouping <code>(1, 11, 06)</code> is invalid because <code>&quot;06&quot;</code> is not a valid code (only <code>&quot;6&quot;</code> is valid).</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>Note: there may be strings that are impossible to decode.<br />
<br />
Given a string s containing only digits, return the <strong>number of ways</strong> to <strong>decode</strong> it. If the entire string cannot be decoded in any valid way, return <code>0</code>.</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 class="example">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>
<div class="example-block">
<p><strong>Input:</strong> <span class="example-io">s = &quot;12&quot;</span></p>
<p><strong>Output:</strong> <span class="example-io">2</span></p>
<p><strong>Explanation:</strong></p>
<p>&quot;12&quot; could be decoded as &quot;AB&quot; (1 2) or &quot;L&quot; (12).</p>
</div>
<p><strong class="example">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>
<div class="example-block">
<p><strong>Input:</strong> <span class="example-io">s = &quot;226&quot;</span></p>
<p><strong>Output:</strong> <span class="example-io">3</span></p>
<p><strong>Explanation:</strong></p>
<p>&quot;226&quot; could be decoded as &quot;BZ&quot; (2 26), &quot;VF&quot; (22 6), or &quot;BBF&quot; (2 2 6).</p>
</div>
<p><strong class="example">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>
<div class="example-block">
<p><strong>Input:</strong> <span class="example-io">s = &quot;06&quot;</span></p>
<p><strong>Output:</strong> <span class="example-io">0</span></p>
<p><strong>Explanation:</strong></p>
<p>&quot;06&quot; cannot be mapped to &quot;F&quot; because of the leading zero (&quot;6&quot; is different from &quot;06&quot;). In this case, the string is not a valid encoding, so return 0.</p>
</div>
<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>