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<a href="https://github.com/krahets/hello-algo/tree/master/docs/chapter_searching/hashing_search.md" title="编辑此页" class="md-content__button md-icon">
<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24"><path d="M20.71 7.04c.39-.39.39-1.04 0-1.41l-2.34-2.34c-.37-.39-1.02-.39-1.41 0l-1.84 1.83 3.75 3.75M3 17.25V21h3.75L17.81 9.93l-3.75-3.75L3 17.25Z"/></svg>
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<h1 id="_1">哈希查找<a class="headerlink" href="#_1" title="Permanent link">&para;</a></h1>
<div class="admonition question">
<p class="admonition-title">Question</p>
<p>在数据量很大时,「线性查找」太慢;而「二分查找」要求数据必须是有序的,并且只能在数组中应用。那么是否有方法可以同时避免上述缺点呢?答案是肯定的,此方法被称为「哈希查找」。</p>
</div>
<p>「哈希查找 Hash Searching」借助一个哈希表来存储需要的「键值对 Key Value Pair」我们可以在 <span class="arithmatex">\(O(1)\)</span> 时间下实现 “键 <span class="arithmatex">\(\rightarrow\)</span> 值” 映射查找,体现着 “以空间换时间” 的算法思想。</p>
<h2 id="_2">算法实现<a class="headerlink" href="#_2" title="Permanent link">&para;</a></h2>
<p>如果我们想要给定数组中的一个目标元素 <code>target</code> ,获取该元素的索引,那么可以借助一个哈希表实现查找。</p>
<p><img alt="hash_search_index" src="../hashing_search.assets/hash_search_index.png" /></p>
<div class="tabbed-set tabbed-alternate" data-tabs="1:8"><input checked="checked" id="__tabbed_1_1" name="__tabbed_1" type="radio" /><input id="__tabbed_1_2" name="__tabbed_1" type="radio" /><input id="__tabbed_1_3" name="__tabbed_1" type="radio" /><input id="__tabbed_1_4" name="__tabbed_1" type="radio" /><input id="__tabbed_1_5" name="__tabbed_1" type="radio" /><input id="__tabbed_1_6" name="__tabbed_1" type="radio" /><input id="__tabbed_1_7" name="__tabbed_1" type="radio" /><input id="__tabbed_1_8" name="__tabbed_1" type="radio" /><div class="tabbed-labels"><label for="__tabbed_1_1">Java</label><label for="__tabbed_1_2">C++</label><label for="__tabbed_1_3">Python</label><label for="__tabbed_1_4">Go</label><label for="__tabbed_1_5">JavaScript</label><label for="__tabbed_1_6">TypeScript</label><label for="__tabbed_1_7">C</label><label for="__tabbed_1_8">C#</label></div>
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<div class="highlight"><span class="filename">hashing_search.java</span><pre><span></span><code><a id="__codelineno-0-1" name="__codelineno-0-1" href="#__codelineno-0-1"></a><span class="cm">/* 哈希查找(数组) */</span><span class="w"></span>
<a id="__codelineno-0-2" name="__codelineno-0-2" href="#__codelineno-0-2"></a><span class="kt">int</span><span class="w"> </span><span class="nf">hashingSearch</span><span class="p">(</span><span class="n">Map</span><span class="o">&lt;</span><span class="n">Integer</span><span class="p">,</span><span class="w"> </span><span class="n">Integer</span><span class="o">&gt;</span><span class="w"> </span><span class="n">map</span><span class="p">,</span><span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">target</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<a id="__codelineno-0-3" name="__codelineno-0-3" href="#__codelineno-0-3"></a><span class="w"> </span><span class="c1">// 哈希表的 key: 目标元素value: 索引</span><span class="w"></span>
<a id="__codelineno-0-4" name="__codelineno-0-4" href="#__codelineno-0-4"></a><span class="w"> </span><span class="c1">// 若哈希表中无此 key ,返回 -1</span><span class="w"></span>
<a id="__codelineno-0-5" name="__codelineno-0-5" href="#__codelineno-0-5"></a><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">map</span><span class="p">.</span><span class="na">getOrDefault</span><span class="p">(</span><span class="n">target</span><span class="p">,</span><span class="w"> </span><span class="o">-</span><span class="mi">1</span><span class="p">);</span><span class="w"></span>
<a id="__codelineno-0-6" name="__codelineno-0-6" href="#__codelineno-0-6"></a><span class="p">}</span><span class="w"></span>
</code></pre></div>
</div>
<div class="tabbed-block">
<div class="highlight"><span class="filename">hashing_search.cpp</span><pre><span></span><code><a id="__codelineno-1-1" name="__codelineno-1-1" href="#__codelineno-1-1"></a><span class="cm">/* 哈希查找(数组) */</span><span class="w"></span>
<a id="__codelineno-1-2" name="__codelineno-1-2" href="#__codelineno-1-2"></a><span class="kt">int</span><span class="w"> </span><span class="nf">hashingSearch</span><span class="p">(</span><span class="n">unordered_map</span><span class="o">&lt;</span><span class="kt">int</span><span class="p">,</span><span class="w"> </span><span class="kt">int</span><span class="o">&gt;</span><span class="w"> </span><span class="n">map</span><span class="p">,</span><span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">target</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<a id="__codelineno-1-3" name="__codelineno-1-3" href="#__codelineno-1-3"></a><span class="w"> </span><span class="c1">// 哈希表的 key: 目标元素value: 索引</span>
<a id="__codelineno-1-4" name="__codelineno-1-4" href="#__codelineno-1-4"></a><span class="w"> </span><span class="c1">// 若哈希表中无此 key ,返回 -1</span>
<a id="__codelineno-1-5" name="__codelineno-1-5" href="#__codelineno-1-5"></a><span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">map</span><span class="p">.</span><span class="n">find</span><span class="p">(</span><span class="n">target</span><span class="p">)</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="n">map</span><span class="p">.</span><span class="n">end</span><span class="p">())</span><span class="w"></span>
<a id="__codelineno-1-6" name="__codelineno-1-6" href="#__codelineno-1-6"></a><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="mi">-1</span><span class="p">;</span><span class="w"></span>
<a id="__codelineno-1-7" name="__codelineno-1-7" href="#__codelineno-1-7"></a><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">map</span><span class="p">[</span><span class="n">target</span><span class="p">];</span><span class="w"></span>
<a id="__codelineno-1-8" name="__codelineno-1-8" href="#__codelineno-1-8"></a><span class="p">}</span><span class="w"></span>
</code></pre></div>
</div>
<div class="tabbed-block">
<div class="highlight"><span class="filename">hashing_search.py</span><pre><span></span><code><a id="__codelineno-2-1" name="__codelineno-2-1" href="#__codelineno-2-1"></a><span class="sd">&quot;&quot;&quot; 哈希查找(数组) &quot;&quot;&quot;</span>
<a id="__codelineno-2-2" name="__codelineno-2-2" href="#__codelineno-2-2"></a><span class="k">def</span> <span class="nf">hashing_search</span><span class="p">(</span><span class="n">mapp</span><span class="p">,</span> <span class="n">target</span><span class="p">):</span>
<a id="__codelineno-2-3" name="__codelineno-2-3" href="#__codelineno-2-3"></a> <span class="c1"># 哈希表的 key: 目标元素value: 索引</span>
<a id="__codelineno-2-4" name="__codelineno-2-4" href="#__codelineno-2-4"></a> <span class="c1"># 若哈希表中无此 key ,返回 -1</span>
<a id="__codelineno-2-5" name="__codelineno-2-5" href="#__codelineno-2-5"></a> <span class="k">return</span> <span class="n">mapp</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="n">target</span><span class="p">,</span> <span class="o">-</span><span class="mi">1</span><span class="p">)</span>
</code></pre></div>
</div>
<div class="tabbed-block">
<div class="highlight"><span class="filename">hashing_search.go</span><pre><span></span><code><a id="__codelineno-3-1" name="__codelineno-3-1" href="#__codelineno-3-1"></a><span class="cm">/* 哈希查找(数组) */</span><span class="w"></span>
<a id="__codelineno-3-2" name="__codelineno-3-2" href="#__codelineno-3-2"></a><span class="kd">func</span><span class="w"> </span><span class="nx">hashingSearch</span><span class="p">(</span><span class="nx">m</span><span class="w"> </span><span class="kd">map</span><span class="p">[</span><span class="kt">int</span><span class="p">]</span><span class="kt">int</span><span class="p">,</span><span class="w"> </span><span class="nx">target</span><span class="w"> </span><span class="kt">int</span><span class="p">)</span><span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<a id="__codelineno-3-3" name="__codelineno-3-3" href="#__codelineno-3-3"></a><span class="c1">// 哈希表的 key: 目标元素value: 索引</span><span class="w"></span>
<a id="__codelineno-3-4" name="__codelineno-3-4" href="#__codelineno-3-4"></a><span class="c1">// 若哈希表中无此 key ,返回 -1</span><span class="w"></span>
<a id="__codelineno-3-5" name="__codelineno-3-5" href="#__codelineno-3-5"></a><span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="nx">index</span><span class="p">,</span><span class="w"> </span><span class="nx">ok</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">target</span><span class="p">];</span><span class="w"> </span><span class="nx">ok</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<a id="__codelineno-3-6" name="__codelineno-3-6" href="#__codelineno-3-6"></a><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="nx">index</span><span class="w"></span>
<a id="__codelineno-3-7" name="__codelineno-3-7" href="#__codelineno-3-7"></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><span class="w"></span>
<a id="__codelineno-3-8" name="__codelineno-3-8" href="#__codelineno-3-8"></a><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="o">-</span><span class="mi">1</span><span class="w"></span>
<a id="__codelineno-3-9" name="__codelineno-3-9" href="#__codelineno-3-9"></a><span class="w"> </span><span class="p">}</span><span class="w"></span>
<a id="__codelineno-3-10" name="__codelineno-3-10" href="#__codelineno-3-10"></a><span class="p">}</span><span class="w"></span>
</code></pre></div>
</div>
<div class="tabbed-block">
<div class="highlight"><span class="filename">hashing_search.js</span><pre><span></span><code><a id="__codelineno-4-1" name="__codelineno-4-1" href="#__codelineno-4-1"></a>
</code></pre></div>
</div>
<div class="tabbed-block">
<div class="highlight"><span class="filename">hashing_search.ts</span><pre><span></span><code><a id="__codelineno-5-1" name="__codelineno-5-1" href="#__codelineno-5-1"></a>
</code></pre></div>
</div>
<div class="tabbed-block">
<div class="highlight"><span class="filename">hashing_search.c</span><pre><span></span><code><a id="__codelineno-6-1" name="__codelineno-6-1" href="#__codelineno-6-1"></a>
</code></pre></div>
</div>
<div class="tabbed-block">
<div class="highlight"><span class="filename">hashing_search.cs</span><pre><span></span><code><a id="__codelineno-7-1" name="__codelineno-7-1" href="#__codelineno-7-1"></a>
</code></pre></div>
</div>
</div>
</div>
<p>再比如,如果我们想要给定一个目标结点值 <code>target</code> ,获取对应的链表结点对象,那么也可以使用哈希查找实现。</p>
<p><img alt="hash_search_listnode" src="../hashing_search.assets/hash_search_listnode.png" /></p>
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<div class="highlight"><span class="filename">hashing_search.java</span><pre><span></span><code><a id="__codelineno-8-1" name="__codelineno-8-1" href="#__codelineno-8-1"></a><span class="cm">/* 哈希查找(链表) */</span><span class="w"></span>
<a id="__codelineno-8-2" name="__codelineno-8-2" href="#__codelineno-8-2"></a><span class="n">ListNode</span><span class="w"> </span><span class="nf">hashingSearch1</span><span class="p">(</span><span class="n">Map</span><span class="o">&lt;</span><span class="n">Integer</span><span class="p">,</span><span class="w"> </span><span class="n">ListNode</span><span class="o">&gt;</span><span class="w"> </span><span class="n">map</span><span class="p">,</span><span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">target</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<a id="__codelineno-8-3" name="__codelineno-8-3" href="#__codelineno-8-3"></a><span class="w"> </span><span class="c1">// 哈希表的 key: 目标结点值value: 结点对象</span><span class="w"></span>
<a id="__codelineno-8-4" name="__codelineno-8-4" href="#__codelineno-8-4"></a><span class="w"> </span><span class="c1">// 若哈希表中无此 key ,返回 null</span><span class="w"></span>
<a id="__codelineno-8-5" name="__codelineno-8-5" href="#__codelineno-8-5"></a><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">map</span><span class="p">.</span><span class="na">getOrDefault</span><span class="p">(</span><span class="n">target</span><span class="p">,</span><span class="w"> </span><span class="kc">null</span><span class="p">);</span><span class="w"></span>
<a id="__codelineno-8-6" name="__codelineno-8-6" href="#__codelineno-8-6"></a><span class="p">}</span><span class="w"></span>
</code></pre></div>
</div>
<div class="tabbed-block">
<div class="highlight"><span class="filename">hashing_search.cpp</span><pre><span></span><code><a id="__codelineno-9-1" name="__codelineno-9-1" href="#__codelineno-9-1"></a><span class="cm">/* 哈希查找(链表) */</span><span class="w"></span>
<a id="__codelineno-9-2" name="__codelineno-9-2" href="#__codelineno-9-2"></a><span class="n">ListNode</span><span class="o">*</span><span class="w"> </span><span class="nf">hashingSearch1</span><span class="p">(</span><span class="n">unordered_map</span><span class="o">&lt;</span><span class="kt">int</span><span class="p">,</span><span class="w"> </span><span class="n">ListNode</span><span class="o">*&gt;</span><span class="w"> </span><span class="n">map</span><span class="p">,</span><span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">target</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<a id="__codelineno-9-3" name="__codelineno-9-3" href="#__codelineno-9-3"></a><span class="w"> </span><span class="c1">// 哈希表的 key: 目标结点值value: 结点对象</span>
<a id="__codelineno-9-4" name="__codelineno-9-4" href="#__codelineno-9-4"></a><span class="w"> </span><span class="c1">// 若哈希表中无此 key ,返回 nullptr</span>
<a id="__codelineno-9-5" name="__codelineno-9-5" href="#__codelineno-9-5"></a><span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">map</span><span class="p">.</span><span class="n">find</span><span class="p">(</span><span class="n">target</span><span class="p">)</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="n">map</span><span class="p">.</span><span class="n">end</span><span class="p">())</span><span class="w"></span>
<a id="__codelineno-9-6" name="__codelineno-9-6" href="#__codelineno-9-6"></a><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="k">nullptr</span><span class="p">;</span><span class="w"></span>
<a id="__codelineno-9-7" name="__codelineno-9-7" href="#__codelineno-9-7"></a><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">map</span><span class="p">[</span><span class="n">target</span><span class="p">];</span><span class="w"></span>
<a id="__codelineno-9-8" name="__codelineno-9-8" href="#__codelineno-9-8"></a><span class="p">}</span><span class="w"></span>
</code></pre></div>
</div>
<div class="tabbed-block">
<div class="highlight"><span class="filename">hashing_search.py</span><pre><span></span><code><a id="__codelineno-10-1" name="__codelineno-10-1" href="#__codelineno-10-1"></a><span class="sd">&quot;&quot;&quot; 哈希查找(链表) &quot;&quot;&quot;</span>
<a id="__codelineno-10-2" name="__codelineno-10-2" href="#__codelineno-10-2"></a><span class="k">def</span> <span class="nf">hashing_search1</span><span class="p">(</span><span class="n">mapp</span><span class="p">,</span> <span class="n">target</span><span class="p">):</span>
<a id="__codelineno-10-3" name="__codelineno-10-3" href="#__codelineno-10-3"></a> <span class="c1"># 哈希表的 key: 目标元素value: 结点对象</span>
<a id="__codelineno-10-4" name="__codelineno-10-4" href="#__codelineno-10-4"></a> <span class="c1"># 若哈希表中无此 key ,返回 -1</span>
<a id="__codelineno-10-5" name="__codelineno-10-5" href="#__codelineno-10-5"></a> <span class="k">return</span> <span class="n">mapp</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="n">target</span><span class="p">,</span> <span class="o">-</span><span class="mi">1</span><span class="p">)</span>
</code></pre></div>
</div>
<div class="tabbed-block">
<div class="highlight"><span class="filename">hashing_search.go</span><pre><span></span><code><a id="__codelineno-11-1" name="__codelineno-11-1" href="#__codelineno-11-1"></a><span class="cm">/* 哈希查找(链表) */</span><span class="w"></span>
<a id="__codelineno-11-2" name="__codelineno-11-2" href="#__codelineno-11-2"></a><span class="kd">func</span><span class="w"> </span><span class="nx">hashingSearch1</span><span class="p">(</span><span class="nx">m</span><span class="w"> </span><span class="kd">map</span><span class="p">[</span><span class="kt">int</span><span class="p">]</span><span class="o">*</span><span class="nx">ListNode</span><span class="p">,</span><span class="w"> </span><span class="nx">target</span><span class="w"> </span><span class="kt">int</span><span class="p">)</span><span class="w"> </span><span class="o">*</span><span class="nx">ListNode</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<a id="__codelineno-11-3" name="__codelineno-11-3" href="#__codelineno-11-3"></a><span class="w"> </span><span class="c1">// 哈希表的 key: 目标结点值value: 结点对象</span><span class="w"></span>
<a id="__codelineno-11-4" name="__codelineno-11-4" href="#__codelineno-11-4"></a><span class="w"> </span><span class="c1">// 若哈希表中无此 key ,返回 nil</span><span class="w"></span>
<a id="__codelineno-11-5" name="__codelineno-11-5" href="#__codelineno-11-5"></a><span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="nx">node</span><span class="p">,</span><span class="w"> </span><span class="nx">ok</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">target</span><span class="p">];</span><span class="w"> </span><span class="nx">ok</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<a id="__codelineno-11-6" name="__codelineno-11-6" href="#__codelineno-11-6"></a><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="nx">node</span><span class="w"></span>
<a id="__codelineno-11-7" name="__codelineno-11-7" href="#__codelineno-11-7"></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><span class="w"></span>
<a id="__codelineno-11-8" name="__codelineno-11-8" href="#__codelineno-11-8"></a><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="kc">nil</span><span class="w"></span>
<a id="__codelineno-11-9" name="__codelineno-11-9" href="#__codelineno-11-9"></a><span class="w"> </span><span class="p">}</span><span class="w"></span>
<a id="__codelineno-11-10" name="__codelineno-11-10" href="#__codelineno-11-10"></a><span class="p">}</span><span class="w"> </span>
</code></pre></div>
</div>
<div class="tabbed-block">
<div class="highlight"><span class="filename">hashing_search.js</span><pre><span></span><code><a id="__codelineno-12-1" name="__codelineno-12-1" href="#__codelineno-12-1"></a>
</code></pre></div>
</div>
<div class="tabbed-block">
<div class="highlight"><span class="filename">hashing_search.ts</span><pre><span></span><code><a id="__codelineno-13-1" name="__codelineno-13-1" href="#__codelineno-13-1"></a>
</code></pre></div>
</div>
<div class="tabbed-block">
<div class="highlight"><span class="filename">hashing_search.c</span><pre><span></span><code><a id="__codelineno-14-1" name="__codelineno-14-1" href="#__codelineno-14-1"></a>
</code></pre></div>
</div>
<div class="tabbed-block">
<div class="highlight"><span class="filename">hashing_search.cs</span><pre><span></span><code><a id="__codelineno-15-1" name="__codelineno-15-1" href="#__codelineno-15-1"></a>
</code></pre></div>
</div>
</div>
</div>
<h2 id="_3">复杂度分析<a class="headerlink" href="#_3" title="Permanent link">&para;</a></h2>
<p><strong>时间复杂度:</strong> <span class="arithmatex">\(O(1)\)</span> ,哈希表的查找操作使用 <span class="arithmatex">\(O(1)\)</span> 时间。</p>
<p><strong>空间复杂度:</strong> <span class="arithmatex">\(O(n)\)</span> ,其中 <span class="arithmatex">\(n\)</span> 为数组或链表长度。</p>
<h2 id="_4">优缺点<a class="headerlink" href="#_4" title="Permanent link">&para;</a></h2>
<p>在哈希表中,<strong>查找、插入、删除操作的平均时间复杂度都为 <span class="arithmatex">\(O(1)\)</span></strong> ,这意味着无论是高频增删还是高频查找场景,哈希查找的性能表现都非常好。当然,一切的前提是保证哈希表未退化。</p>
<p>即使如此,哈希查找仍存在一些问题,在实际应用中,需要根据情况灵活选择方法。</p>
<ul>
<li>辅助哈希表 <strong>需要使用 <span class="arithmatex">\(O(n)\)</span> 的额外空间</strong>,意味着需要预留更多的计算机内存;</li>
<li>建立和维护哈希表需要时间,因此哈希查找 <strong>不适合高频增删、低频查找的使用场景</strong></li>
<li>当哈希冲突严重时,哈希表会退化为链表,<strong>时间复杂度劣化至 <span class="arithmatex">\(O(n)\)</span></strong> </li>
<li><strong>当数据量很小时,线性查找比哈希查找更快</strong>。这是因为计算哈希映射函数可能比遍历一个小型数组更慢;</li>
</ul>
<h2 id="__comments">评论</h2>
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