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leetcode-cn/problem (Chinese)/统计元音字母序列的数目 [count-vowels-permutation].html Normal file
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<p>给你一个整数&nbsp;<code>n</code>，请你帮忙统计一下我们可以按下述规则形成多少个长度为&nbsp;<code>n</code>&nbsp;的字符串：</p>
<ul>
<li>字符串中的每个字符都应当是小写元音字母（<code>&#39;a&#39;</code>, <code>&#39;e&#39;</code>, <code>&#39;i&#39;</code>, <code>&#39;o&#39;</code>, <code>&#39;u&#39;</code>）</li>
<li>每个元音&nbsp;<code>&#39;a&#39;</code>&nbsp;后面都只能跟着&nbsp;<code>&#39;e&#39;</code></li>
<li>每个元音&nbsp;<code>&#39;e&#39;</code>&nbsp;后面只能跟着&nbsp;<code>&#39;a&#39;</code>&nbsp;或者是&nbsp;<code>&#39;i&#39;</code></li>
<li>每个元音&nbsp;<code>&#39;i&#39;</code>&nbsp;后面&nbsp;<strong>不能</strong> 再跟着另一个&nbsp;<code>&#39;i&#39;</code></li>
<li>每个元音&nbsp;<code>&#39;o&#39;</code>&nbsp;后面只能跟着&nbsp;<code>&#39;i&#39;</code>&nbsp;或者是&nbsp;<code>&#39;u&#39;</code></li>
<li>每个元音&nbsp;<code>&#39;u&#39;</code>&nbsp;后面只能跟着&nbsp;<code>&#39;a&#39;</code></li>
</ul>
<p>由于答案可能会很大，所以请你返回 模&nbsp;<code>10^9 + 7</code>&nbsp;之后的结果。</p>
<p>&nbsp;</p>
<p><strong>示例 1：</strong></p>
<pre><strong>输入：</strong>n = 1
<strong>输出：</strong>5
<strong>解释：</strong>所有可能的字符串分别是：&quot;a&quot;, &quot;e&quot;, &quot;i&quot; , &quot;o&quot; 和 &quot;u&quot;。
</pre>
<p><strong>示例 2：</strong></p>
<pre><strong>输入：</strong>n = 2
<strong>输出：</strong>10
<strong>解释：</strong>所有可能的字符串分别是：&quot;ae&quot;, &quot;ea&quot;, &quot;ei&quot;, &quot;ia&quot;, &quot;ie&quot;, &quot;io&quot;, &quot;iu&quot;, &quot;oi&quot;, &quot;ou&quot; 和 &quot;ua&quot;。
</pre>
<p><strong>示例 3：</strong></p>
<pre><strong>输入：</strong>n = 5
<strong>输出：</strong>68</pre>
<p>&nbsp;</p>
<p><strong>提示：</strong></p>
<ul>
<li><code>1 &lt;= n &lt;= 2 * 10^4</code></li>
</ul>