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<h1 id="61">6.1 哈希表<a class="headerlink" href="#61" title="Permanent link">¶</a></h1>
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<p>「哈希表 hash table」，又称「散列表」，其通过建立键 <code>key</code> 与值 <code>value</code> 之间的映射，实现高效的元素查询。具体而言，我们向哈希表输入一个键 <code>key</code> ，则可以在 <span class="arithmatex">\(O(1)\)</span> 时间内获取对应的值 <code>value</code> 。</p>
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<p>如图 6-1 所示，给定 <span class="arithmatex">\(n\)</span> 个学生，每个学生都有“姓名”和“学号”两项数据。假如我们希望实现“输入一个学号，返回对应的姓名”的查询功能，则可以采用图 6-1 所示的哈希表来实现。</p>
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@ -4543,11 +4575,10 @@
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<a id="__codelineno-31-77" name="__codelineno-31-77" href="#__codelineno-31-77"></a><span class="w"> </span><span class="nx">print</span><span class="p">()</span><span class="w"> </span><span class="p">{</span>
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<a id="__codelineno-31-78" name="__codelineno-31-78" href="#__codelineno-31-78"></a><span class="w"> </span><span class="kd">let</span><span class="w"> </span><span class="nx">pairSet</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">this</span><span class="p">.</span><span class="nx">entries</span><span class="p">();</span>
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<a id="__codelineno-31-79" name="__codelineno-31-79" href="#__codelineno-31-79"></a><span class="w"> </span><span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="kd">const</span><span class="w"> </span><span class="nx">pair</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="nx">pairSet</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
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<a id="__codelineno-31-80" name="__codelineno-31-80" href="#__codelineno-31-80"></a><span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="o">!</span><span class="nx">pair</span><span class="p">)</span><span class="w"> </span><span class="k">continue</span><span class="p">;</span>
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<a id="__codelineno-31-81" name="__codelineno-31-81" href="#__codelineno-31-81"></a><span class="w"> </span><span class="nx">console</span><span class="p">.</span><span class="nx">info</span><span class="p">(</span><span class="sb">`</span><span class="si">${</span><span class="nx">pair</span><span class="p">.</span><span class="nx">key</span><span class="si">}</span><span class="sb"> -> </span><span class="si">${</span><span class="nx">pair</span><span class="p">.</span><span class="nx">val</span><span class="si">}</span><span class="sb">`</span><span class="p">);</span>
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<a id="__codelineno-31-82" name="__codelineno-31-82" href="#__codelineno-31-82"></a><span class="w"> </span><span class="p">}</span>
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<a id="__codelineno-31-83" name="__codelineno-31-83" href="#__codelineno-31-83"></a><span class="w"> </span><span class="p">}</span>
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<a id="__codelineno-31-84" name="__codelineno-31-84" href="#__codelineno-31-84"></a><span class="p">}</span>
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<a id="__codelineno-31-80" name="__codelineno-31-80" href="#__codelineno-31-80"></a><span class="w"> </span><span class="nx">console</span><span class="p">.</span><span class="nx">info</span><span class="p">(</span><span class="sb">`</span><span class="si">${</span><span class="nx">pair</span><span class="p">.</span><span class="nx">key</span><span class="si">}</span><span class="sb"> -> </span><span class="si">${</span><span class="nx">pair</span><span class="p">.</span><span class="nx">val</span><span class="si">}</span><span class="sb">`</span><span class="p">);</span>
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<a id="__codelineno-31-81" name="__codelineno-31-81" href="#__codelineno-31-81"></a><span class="w"> </span><span class="p">}</span>
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<a id="__codelineno-31-82" name="__codelineno-31-82" href="#__codelineno-31-82"></a><span class="w"> </span><span class="p">}</span>
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<a id="__codelineno-31-83" name="__codelineno-31-83" href="#__codelineno-31-83"></a><span class="p">}</span>
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</code></pre></div>
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</div>
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<div class="tabbed-block">
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@ -4634,11 +4665,10 @@
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<a id="__codelineno-32-81" name="__codelineno-32-81" href="#__codelineno-32-81"></a><span class="w"> </span><span class="k">public</span><span class="w"> </span><span class="nx">print</span><span class="p">()</span><span class="w"> </span><span class="p">{</span>
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<a id="__codelineno-32-82" name="__codelineno-32-82" href="#__codelineno-32-82"></a><span class="w"> </span><span class="kd">let</span><span class="w"> </span><span class="nx">pairSet</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">this</span><span class="p">.</span><span class="nx">entries</span><span class="p">();</span>
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<a id="__codelineno-32-83" name="__codelineno-32-83" href="#__codelineno-32-83"></a><span class="w"> </span><span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="kd">const</span><span class="w"> </span><span class="nx">pair</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="nx">pairSet</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
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<a id="__codelineno-32-84" name="__codelineno-32-84" href="#__codelineno-32-84"></a><span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="o">!</span><span class="nx">pair</span><span class="p">)</span><span class="w"> </span><span class="k">continue</span><span class="p">;</span>
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<a id="__codelineno-32-85" name="__codelineno-32-85" href="#__codelineno-32-85"></a><span class="w"> </span><span class="nx">console</span><span class="p">.</span><span class="nx">info</span><span class="p">(</span><span class="sb">`</span><span class="si">${</span><span class="nx">pair</span><span class="p">.</span><span class="nx">key</span><span class="si">}</span><span class="sb"> -> </span><span class="si">${</span><span class="nx">pair</span><span class="p">.</span><span class="nx">val</span><span class="si">}</span><span class="sb">`</span><span class="p">);</span>
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<a id="__codelineno-32-86" name="__codelineno-32-86" href="#__codelineno-32-86"></a><span class="w"> </span><span class="p">}</span>
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<a id="__codelineno-32-87" name="__codelineno-32-87" href="#__codelineno-32-87"></a><span class="w"> </span><span class="p">}</span>
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<a id="__codelineno-32-88" name="__codelineno-32-88" href="#__codelineno-32-88"></a><span class="p">}</span>
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<a id="__codelineno-32-84" name="__codelineno-32-84" href="#__codelineno-32-84"></a><span class="w"> </span><span class="nx">console</span><span class="p">.</span><span class="nx">info</span><span class="p">(</span><span class="sb">`</span><span class="si">${</span><span class="nx">pair</span><span class="p">.</span><span class="nx">key</span><span class="si">}</span><span class="sb"> -> </span><span class="si">${</span><span class="nx">pair</span><span class="p">.</span><span class="nx">val</span><span class="si">}</span><span class="sb">`</span><span class="p">);</span>
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<a id="__codelineno-32-85" name="__codelineno-32-85" href="#__codelineno-32-85"></a><span class="w"> </span><span class="p">}</span>
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<a id="__codelineno-32-86" name="__codelineno-32-86" href="#__codelineno-32-86"></a><span class="w"> </span><span class="p">}</span>
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<a id="__codelineno-32-87" name="__codelineno-32-87" href="#__codelineno-32-87"></a><span class="p">}</span>
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</code></pre></div>
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</div>
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<div class="tabbed-block">
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@ -4937,11 +4967,73 @@
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<p>类似于数组扩容，哈希表扩容需将所有键值对从原哈希表迁移至新哈希表，非常耗时。并且由于哈希表容量 <code>capacity</code> 改变，我们需要通过哈希函数来重新计算所有键值对的存储位置，这进一步提高了扩容过程的计算开销。为此，编程语言通常会预留足够大的哈希表容量，防止频繁扩容。</p>
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<p>「负载因子 load factor」是哈希表的一个重要概念，其定义为哈希表的元素数量除以桶数量，用于衡量哈希冲突的严重程度，<strong>也常被作为哈希表扩容的触发条件</strong>。例如在 Java 中，当负载因子超过 <span class="arithmatex">\(0.75\)</span> 时，系统会将哈希表容量扩展为原先的 <span class="arithmatex">\(2\)</span> 倍。</p>
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