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<p class="admonition-title">快速排序的分治思想</p>
<p>哨兵划分的实质是将一个较长数组的排序问题简化为两个较短数组的排序问题。</p>
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<div class="highlight"><span class="filename">quick_sort.java</span><pre><span></span><code><a id="__codelineno-0-1" name="__codelineno-0-1" href="#__codelineno-0-1"></a><span class="cm">/* 元素交换 */</span>
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<p><img alt="快速排序流程" src="../quick_sort.assets/quick_sort_overview.png" /></p>
<p align="center"> Fig. 快速排序流程 </p>
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<div class="highlight"><span class="filename">quick_sort.java</span><pre><span></span><code><a id="__codelineno-12-1" name="__codelineno-12-1" href="#__codelineno-12-1"></a><span class="cm">/* 快速排序 */</span>
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<p>为了尽量避免这种情况发生，<strong>我们可以优化哨兵划分中的基准数的选取策略</strong>。例如，我们可以随机选取一个元素作为基准数。然而，如果运气不佳，每次都选到不理想的基准数，效率仍然不尽如人意。</p>
<p>需要注意的是，编程语言通常生成的是“伪随机数”。如果我们针对伪随机数序列构建一个特定的测试样例，那么快速排序的效率仍然可能劣化。</p>
<p>为了进一步改进，我们可以在数组中选取三个候选元素（通常为数组的首、尾、中点元素），<strong>并将这三个候选元素的中位数作为基准数</strong>。这样一来，基准数“既不太小也不太大”的概率将大幅提升。当然，我们还可以选取更多候选元素，以进一步提高算法的稳健性。采用这种方法后，时间复杂度劣化至 <span class="arithmatex">\(O(n^2)\)</span> 的概率大大降低。</p>
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<div class="highlight"><span class="filename">quick_sort.java</span><pre><span></span><code><a id="__codelineno-24-1" name="__codelineno-24-1" href="#__codelineno-24-1"></a><span class="cm">/* 选取三个元素的中位数 */</span>
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<h2 id="1155">11.5.5. &nbsp; 尾递归优化<a class="headerlink" href="#1155" title="Permanent link">&para;</a></h2>
<p><strong>在某些输入下，快速排序可能占用空间较多</strong>。以完全倒序的输入数组为例，由于每轮哨兵划分后右子数组长度为 <span class="arithmatex">\(0\)</span> ，递归树的高度会达到 <span class="arithmatex">\(n - 1\)</span> ，此时需要占用 <span class="arithmatex">\(O(n)\)</span> 大小的栈帧空间。</p>
<p>为了防止栈帧空间的累积，我们可以在每轮哨兵排序完成后，比较两个子数组的长度，<strong>仅对较短的子数组进行递归</strong>。由于较短子数组的长度不会超过 <span class="arithmatex">\(\frac{n}{2}\)</span> ，因此这种方法能确保递归深度不超过 <span class="arithmatex">\(\log n\)</span> ，从而将最差空间复杂度优化至 <span class="arithmatex">\(O(\log n)\)</span> 。</p>
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<div class="highlight"><span class="filename">quick_sort.java</span><pre><span></span><code><a id="__codelineno-36-1" name="__codelineno-36-1" href="#__codelineno-36-1"></a><span class="cm">/* 快速排序（尾递归优化） */</span>