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chapter_hashing/hash_algorithm/index.html
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</tbody>
</table>
<h2 id="634">6.3.4. &nbsp; 数据结构的哈希值<a class="headerlink" href="#634" title="Permanent link">&para;</a></h2>
<p>我们知道，哈希表的 <code>key</code> 可以是整数、小数或字符串等数据类型。编程语言通常会为这些数据类型提供内置的哈希算法 <code>hash()</code> ，用于计算哈希表中的桶索引。以 Python 为例：</p>
<p>我们知道，哈希表的 <code>key</code> 可以是整数、小数或字符串等数据类型。编程语言通常会为这些数据类型提供内置的哈希算法，用于计算哈希表中的桶索引。以 Python 为例，我们可以调用 <code>hash()</code> 函数来计算各种数据类型的哈希值，包括：</p>
<ul>
<li>整数和布尔量的哈希值就是其本身。</li>
<li>浮点数和字符串的哈希值计算较为复杂，有兴趣的同学请自行学习。</li>

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chapter_hashing/summary/index.html
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<h1 id="64">6.4. &nbsp; 小结<a class="headerlink" href="#64" title="Permanent link">&para;</a></h1>
<ul>
<li>哈希表能够在 <span class="arithmatex">\(O(1)\)</span> 时间内将键 key 映射到值 value，效率非常高。</li>
<li>常见的哈希表操作包括查询、添加与删除键值对、遍历键值对等。</li>
<li>哈希函数将 key 映射为数组索引（桶索引），从而访问对应的值 value 。</li>
<li>两个不同的 key 可能在经过哈希函数后得到相同的索引，导致查询结果出错，这种现象被称为哈希冲突。</li>
<li>缓解哈希冲突的方法主要有扩容哈希表和优化哈希表的表示方法。</li>
<li>负载因子定义为哈希表中元素数量除以桶数量，反映了哈希冲突的严重程度，常用作触发哈希表扩容的条件。与数组扩容类似，哈希表扩容操作也会产生较大的开销。</li>
<li>链式地址通过将单个元素转化为链表，将所有冲突元素存储在同一个链表中，从而解决哈希冲突。然而，过长的链表会降低查询效率，可以通过将链表转换为 AVL 树或红黑树来改善。</li>
<li>开放寻址通过多次探测来解决哈希冲突。线性探测使用固定步长，缺点是不能删除元素且容易产生聚集。多次哈希使用多个哈希函数进行探测，相对线性探测不易产生聚集，但多个哈希函数增加了计算量。</li>
<li>不同编程语言采取了不同的哈希表实现策略。例如，Java 的 HashMap 使用链式地址，而 Python 的 Dict 采用开放寻址。</li>
<li>输入 <code>key</code> ，哈希表能够在 <span class="arithmatex">\(O(1)\)</span> 时间内查询到 <code>value</code> ，效率非常高。</li>
<li>常见的哈希表操作包括查询、添加键值对、删除键值对和遍历哈希表等。</li>
<li>哈希函数将 <code>key</code> 映射为数组索引，从而访问对应桶并获取 <code>value</code> 。</li>
<li>两个不同的 <code>key</code> 可能在经过哈希函数后得到相同的数组索引，导致查询结果出错，这种现象被称为哈希冲突。</li>
<li>哈希表容量越大，哈希冲突的概率就越低。因此可以通过扩容哈希表来缓解哈希冲突。与数组扩容类似，哈希表扩容操作的开销很大。</li>
<li>负载因子定义为哈希表中元素数量除以桶数量，反映了哈希冲突的严重程度，常用作触发哈希表扩容的条件。</li>
<li>链式地址通过将单个元素转化为链表，将所有冲突元素存储在同一个链表中。然而，链表过长会降低查询效率，可以进一步将链表转换为红黑树来提高效率。</li>
<li>开放寻址通过多次探测来处理哈希冲突。线性探测使用固定步长，缺点是不能删除元素，且容易产生聚集。多次哈希使用多个哈希函数进行探测，相较线性探测更不易产生聚集，但多个哈希函数增加了计算量。</li>
<li>不同编程语言采取了不同的哈希表实现。例如，Java 的 <code>HashMap</code> 使用链式地址，而 Python 的 <code>Dict</code> 采用开放寻址。</li>
<li>在哈希表中，我们希望哈希算法具有确定性、高效率和均匀分布的特点。在密码学中，哈希算法还应该具备抗碰撞性和雪崩效应。</li>
<li>哈希算法通常采用大质数作为模数，以最大化地保证哈希值的均匀分布，减少哈希冲突。</li>
<li>常见的哈希算法包括 MD5, SHA-1, SHA-2, SHA3 等。MD5 常用语校验文件完整性，SHA-2 常用于安全应用与协议。</li>
<li>编程语言通常会为数据类型提供内置哈希算法，用于计算哈希表中的桶索引。通常情况下，只有不可变对象是可哈希的。</li>
</ul>

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chapter_searching/replace_linear_by_hashing/index.html
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@ -2376,10 +2376,10 @@
<a id="__codelineno-15-4" name="__codelineno-15-4" href="#__codelineno-15-4"></a><span class="w"> </span><span class="kd">let</span><span class="w"> </span><span class="nx">m</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{};</span>
<a id="__codelineno-15-5" name="__codelineno-15-5" href="#__codelineno-15-5"></a><span class="w"> </span><span class="c1">// 单层循环，时间复杂度 O(n)</span>
<a id="__codelineno-15-6" name="__codelineno-15-6" href="#__codelineno-15-6"></a><span class="w"> </span><span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="kd">let</span><span class="w"> </span><span class="nx">i</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mf">0</span><span class="p">;</span><span class="w"> </span><span class="nx">i</span><span class="w"> </span><span class="o">&lt;</span><span class="w"> </span><span class="nx">nums</span><span class="p">.</span><span class="nx">length</span><span class="p">;</span><span class="w"> </span><span class="nx">i</span><span class="o">++</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<a id="__codelineno-15-7" name="__codelineno-15-7" href="#__codelineno-15-7"></a><span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="nx">m</span><span class="p">[</span><span class="nx">nums</span><span class="p">[</span><span class="nx">i</span><span class="p">]]</span><span class="w"> </span><span class="o">!==</span><span class="w"> </span><span class="kc">undefined</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<a id="__codelineno-15-8" name="__codelineno-15-8" href="#__codelineno-15-8"></a><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="p">[</span><span class="nx">m</span><span class="p">[</span><span class="nx">nums</span><span class="p">[</span><span class="nx">i</span><span class="p">]],</span><span class="w"> </span><span class="nx">i</span><span class="p">];</span>
<a id="__codelineno-15-7" name="__codelineno-15-7" href="#__codelineno-15-7"></a><span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="nx">m</span><span class="p">[</span><span class="nx">target</span><span class="w"> </span><span class="o">-</span><span class="w"> </span><span class="nx">nums</span><span class="p">[</span><span class="nx">i</span><span class="p">]]</span><span class="w"> </span><span class="o">!==</span><span class="w"> </span><span class="kc">undefined</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<a id="__codelineno-15-8" name="__codelineno-15-8" href="#__codelineno-15-8"></a><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="p">[</span><span class="nx">m</span><span class="p">[</span><span class="nx">target</span><span class="o">-</span><span class="nx">nums</span><span class="p">[</span><span class="nx">i</span><span class="p">]],</span><span class="w"> </span><span class="nx">i</span><span class="p">];</span>
<a id="__codelineno-15-9" name="__codelineno-15-9" href="#__codelineno-15-9"></a><span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="k">else</span><span class="w"> </span><span class="p">{</span>
<a id="__codelineno-15-10" name="__codelineno-15-10" href="#__codelineno-15-10"></a><span class="w"> </span><span class="nx">m</span><span class="p">[</span><span class="nx">target</span><span class="w"> </span><span class="o">-</span><span class="w"> </span><span class="nx">nums</span><span class="p">[</span><span class="nx">i</span><span class="p">]]</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="nx">i</span><span class="p">;</span>
<a id="__codelineno-15-10" name="__codelineno-15-10" href="#__codelineno-15-10"></a><span class="w"> </span><span class="nx">m</span><span class="p">[</span><span class="nx">nums</span><span class="p">[</span><span class="nx">i</span><span class="p">]]</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="nx">i</span><span class="p">;</span>
<a id="__codelineno-15-11" name="__codelineno-15-11" href="#__codelineno-15-11"></a><span class="w"> </span><span class="p">}</span>
<a id="__codelineno-15-12" name="__codelineno-15-12" href="#__codelineno-15-12"></a><span class="w"> </span><span class="p">}</span>
<a id="__codelineno-15-13" name="__codelineno-15-13" href="#__codelineno-15-13"></a><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="p">[];</span>
@ -2393,11 +2393,11 @@
<a id="__codelineno-16-4" name="__codelineno-16-4" href="#__codelineno-16-4"></a><span class="w"> </span><span class="kd">let</span><span class="w"> </span><span class="nx">m</span><span class="o">:</span><span class="w"> </span><span class="kt">Map</span><span class="o">&lt;</span><span class="kt">number</span><span class="p">,</span><span class="w"> </span><span class="kt">number</span><span class="o">&gt;</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="ow">new</span><span class="w"> </span><span class="nb">Map</span><span class="p">();</span>
<a id="__codelineno-16-5" name="__codelineno-16-5" href="#__codelineno-16-5"></a><span class="w"> </span><span class="c1">// 单层循环，时间复杂度 O(n)</span>
<a id="__codelineno-16-6" name="__codelineno-16-6" href="#__codelineno-16-6"></a><span class="w"> </span><span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="kd">let</span><span class="w"> </span><span class="nx">i</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mf">0</span><span class="p">;</span><span class="w"> </span><span class="nx">i</span><span class="w"> </span><span class="o">&lt;</span><span class="w"> </span><span class="nx">nums</span><span class="p">.</span><span class="nx">length</span><span class="p">;</span><span class="w"> </span><span class="nx">i</span><span class="o">++</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<a id="__codelineno-16-7" name="__codelineno-16-7" href="#__codelineno-16-7"></a><span class="w"> </span><span class="kd">let</span><span class="w"> </span><span class="nx">index</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="nx">m</span><span class="p">.</span><span class="nx">get</span><span class="p">(</span><span class="nx">nums</span><span class="p">[</span><span class="nx">i</span><span class="p">]);</span>
<a id="__codelineno-16-7" name="__codelineno-16-7" href="#__codelineno-16-7"></a><span class="w"> </span><span class="kd">let</span><span class="w"> </span><span class="nx">index</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="nx">m</span><span class="p">.</span><span class="nx">get</span><span class="p">(</span><span class="nx">target</span><span class="w"> </span><span class="o">-</span><span class="w"> </span><span class="nx">nums</span><span class="p">[</span><span class="nx">i</span><span class="p">]);</span>
<a id="__codelineno-16-8" name="__codelineno-16-8" href="#__codelineno-16-8"></a><span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="nx">index</span><span class="w"> </span><span class="o">!==</span><span class="w"> </span><span class="kc">undefined</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<a id="__codelineno-16-9" name="__codelineno-16-9" href="#__codelineno-16-9"></a><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="p">[</span><span class="nx">index</span><span class="p">,</span><span class="w"> </span><span class="nx">i</span><span class="p">];</span>
<a id="__codelineno-16-10" name="__codelineno-16-10" href="#__codelineno-16-10"></a><span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="k">else</span><span class="w"> </span><span class="p">{</span>
<a id="__codelineno-16-11" name="__codelineno-16-11" href="#__codelineno-16-11"></a><span class="w"> </span><span class="nx">m</span><span class="p">.</span><span class="nx">set</span><span class="p">(</span><span class="nx">target</span><span class="w"> </span><span class="o">-</span><span class="w"> </span><span class="nx">nums</span><span class="p">[</span><span class="nx">i</span><span class="p">],</span><span class="w"> </span><span class="nx">i</span><span class="p">);</span>
<a id="__codelineno-16-11" name="__codelineno-16-11" href="#__codelineno-16-11"></a><span class="w"> </span><span class="nx">m</span><span class="p">.</span><span class="nx">set</span><span class="p">(</span><span class="nx">nums</span><span class="p">[</span><span class="nx">i</span><span class="p">],</span><span class="w"> </span><span class="nx">i</span><span class="p">);</span>
<a id="__codelineno-16-12" name="__codelineno-16-12" href="#__codelineno-16-12"></a><span class="w"> </span><span class="p">}</span>
<a id="__codelineno-16-13" name="__codelineno-16-13" href="#__codelineno-16-13"></a><span class="w"> </span><span class="p">}</span>
<a id="__codelineno-16-14" name="__codelineno-16-14" href="#__codelineno-16-14"></a><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="p">[];</span>

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