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<h1 id="63">6.3 &nbsp; 哈希算法<a class="headerlink" href="#63" title="Permanent link">&para;</a></h1>
<p>在上两节中,我们了解了哈希表的工作原理和哈希冲突的处理方法。然而无论是开放寻址还是链地址法,<strong>它们只能保证哈希表可以在发生冲突时正常工作,但无法减少哈希冲突的发生</strong></p>
<p>如果哈希冲突过于频繁,哈希表的性能则会急剧劣化。如图 6-7 所示,对于链地址哈希表,理想情况下键值对平均分布在各个桶中,达到最佳查询效率;最差情况下所有键值对都被存储到同一个桶中,时间复杂度退化至 <span class="arithmatex">\(O(n)\)</span></p>
<p>如果哈希冲突过于频繁,哈希表的性能则会急剧劣化。如图 6-8 所示,对于链地址哈希表,理想情况下键值对平均分布在各个桶中,达到最佳查询效率;最差情况下所有键值对都被存储到同一个桶中,时间复杂度退化至 <span class="arithmatex">\(O(n)\)</span></p>
<p><img alt="哈希冲突的最佳与最差情况" src="../hash_algorithm.assets/hash_collision_best_worst_condition.png" /></p>
<p align="center"> 图 6-7 &nbsp; 哈希冲突的最佳与最差情况 </p>
<p align="center"> 图 6-8 &nbsp; 哈希冲突的最佳与最差情况 </p>
<p><strong>键值对的分布情况由哈希函数决定</strong>。回忆哈希函数的计算步骤,先计算哈希值,再对数组长度取模:</p>
<div class="highlight"><pre><span></span><code><a id="__codelineno-0-1" name="__codelineno-0-1" href="#__codelineno-0-1"></a><span class="nv">index</span><span class="w"> </span><span class="o">=</span><span class="w"> </span>hash<span class="o">(</span>key<span class="o">)</span><span class="w"> </span>%<span class="w"> </span>capacity
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<p>虽然自定义对象(比如链表节点)的成员变量是可变的,但它是可哈希的。<strong>这是因为对象的哈希值通常是基于内存地址生成的</strong>,即使对象的内容发生了变化,但它的内存地址不变,哈希值仍然是不变的。</p>
<p>细心的你可能发现在不同控制台中运行程序时,输出的哈希值是不同的。<strong>这是因为 Python 解释器在每次启动时都会为字符串哈希函数加入一个随机的盐Salt</strong>。这种做法可以有效防止 HashDoS 攻击,提升哈希算法的安全性。</p>
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