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<p>「二分查找 Binary Search」是一种基于分治思想的高效搜索算法。它利用数据的有序性,每轮减少一半搜索范围,直至找到目标元素或搜索区间为空为止。</p>
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<p>我们先来求解一个简单的二分查找问题。</p>
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<div class="admonition question">
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<p class="admonition-title">给定一个长度为 <span class="arithmatex">\(n\)</span> 的有序数组 <code>nums</code> ,元素按从小到大的顺序排列。查找并返回元素 <code>target</code> 在该数组中的索引。若数组中不包含该元素,则返回 <span class="arithmatex">\(-1\)</span> 。数组中不包含重复元素。</p>
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<p class="admonition-title">Question</p>
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<p>给定一个长度为 <span class="arithmatex">\(n\)</span> 的有序数组 <code>nums</code> ,元素按从小到大的顺序排列。请查找并返回元素 <code>target</code> 在该数组中的索引。若数组中不包含该元素,则返回 <span class="arithmatex">\(-1\)</span> 。数组中不包含重复元素。</p>
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</div>
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<p>该数组的索引范围可以使用区间 <span class="arithmatex">\([0, n - 1]\)</span> 来表示。其中,<strong>中括号表示“闭区间”,即包含边界值本身</strong>。在该表示下,区间 <span class="arithmatex">\([i, j]\)</span> 在 <span class="arithmatex">\(i = j\)</span> 时仍包含一个元素,在 <span class="arithmatex">\(i > j\)</span> 时为空区间。</p>
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<p>接下来,我们基于上述区间定义实现二分查找。先初始化指针 <span class="arithmatex">\(i = 0\)</span> 和 <span class="arithmatex">\(j = n - 1\)</span> ,分别指向数组首元素和尾元素。之后循环执行以下两个步骤:</p>
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<h2 id="1021">10.2.1. 查找最左一个元素<a class="headerlink" href="#1021" title="Permanent link">¶</a></h2>
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<div class="admonition question">
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<p class="admonition-title">查找并返回元素 <code>target</code> 在有序数组 <code>nums</code> 中首次出现的索引。若数组中不包含该元素,则返回 <span class="arithmatex">\(-1\)</span> 。数组可能包含重复元素。</p>
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<p class="admonition-title">Question</p>
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<p>给定一个长度为 <span class="arithmatex">\(n\)</span> 的有序数组 <code>nums</code> 。请查找并返回元素 <code>target</code> 在该数组中首次出现的索引。若数组中不包含该元素,则返回 <span class="arithmatex">\(-1\)</span> 。数组可能包含重复元素。</p>
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</div>
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<p>实际上,我们可以仅通过二分查找解决以上问题。方法的整体框架不变,先计算中点索引 <code>m</code> ,再判断 <code>target</code> 和 <code>nums[m]</code> 大小关系:</p>
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<ul>
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<a id="__codelineno-2-5" name="__codelineno-2-5" href="#__codelineno-2-5"></a> <span class="k">while</span> <span class="n">i</span> <span class="o"><=</span> <span class="n">j</span><span class="p">:</span>
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<a id="__codelineno-2-6" name="__codelineno-2-6" href="#__codelineno-2-6"></a> <span class="n">m</span> <span class="o">=</span> <span class="p">(</span><span class="n">i</span> <span class="o">+</span> <span class="n">j</span><span class="p">)</span> <span class="o">//</span> <span class="mi">2</span> <span class="c1"># 计算中点索引 m</span>
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<a id="__codelineno-2-7" name="__codelineno-2-7" href="#__codelineno-2-7"></a> <span class="k">if</span> <span class="n">nums</span><span class="p">[</span><span class="n">m</span><span class="p">]</span> <span class="o"><</span> <span class="n">target</span><span class="p">:</span>
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<a id="__codelineno-2-8" name="__codelineno-2-8" href="#__codelineno-2-8"></a> <span class="n">i</span> <span class="o">=</span> <span class="n">m</span> <span class="o">+</span> <span class="mi">1</span> <span class="c1"># 此情况说明 target 在区间 [m+1, j] 中</span>
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<a id="__codelineno-2-8" name="__codelineno-2-8" href="#__codelineno-2-8"></a> <span class="n">i</span> <span class="o">=</span> <span class="n">m</span> <span class="o">+</span> <span class="mi">1</span> <span class="c1"># target 在区间 [m+1, j] 中</span>
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<a id="__codelineno-2-9" name="__codelineno-2-9" href="#__codelineno-2-9"></a> <span class="k">elif</span> <span class="n">nums</span><span class="p">[</span><span class="n">m</span><span class="p">]</span> <span class="o">></span> <span class="n">target</span><span class="p">:</span>
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<a id="__codelineno-2-10" name="__codelineno-2-10" href="#__codelineno-2-10"></a> <span class="n">j</span> <span class="o">=</span> <span class="n">m</span> <span class="o">-</span> <span class="mi">1</span> <span class="c1"># 此情况说明 target 在区间 [i, m-1] 中</span>
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<a id="__codelineno-2-10" name="__codelineno-2-10" href="#__codelineno-2-10"></a> <span class="n">j</span> <span class="o">=</span> <span class="n">m</span> <span class="o">-</span> <span class="mi">1</span> <span class="c1"># target 在区间 [i, m-1] 中</span>
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<a id="__codelineno-2-11" name="__codelineno-2-11" href="#__codelineno-2-11"></a> <span class="k">else</span><span class="p">:</span>
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<a id="__codelineno-2-12" name="__codelineno-2-12" href="#__codelineno-2-12"></a> <span class="n">j</span> <span class="o">=</span> <span class="n">m</span> <span class="o">-</span> <span class="mi">1</span> <span class="c1"># 此情况说明首个小于 target 的元素在区间 [i, m-1] 中</span>
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<a id="__codelineno-2-12" name="__codelineno-2-12" href="#__codelineno-2-12"></a> <span class="n">j</span> <span class="o">=</span> <span class="n">m</span> <span class="o">-</span> <span class="mi">1</span> <span class="c1"># 首个小于 target 的元素在区间 [i, m-1] 中</span>
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<a id="__codelineno-2-13" name="__codelineno-2-13" href="#__codelineno-2-13"></a> <span class="k">if</span> <span class="n">i</span> <span class="o">==</span> <span class="nb">len</span><span class="p">(</span><span class="n">nums</span><span class="p">)</span> <span class="ow">or</span> <span class="n">nums</span><span class="p">[</span><span class="n">i</span><span class="p">]</span> <span class="o">!=</span> <span class="n">target</span><span class="p">:</span>
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<a id="__codelineno-2-14" name="__codelineno-2-14" href="#__codelineno-2-14"></a> <span class="k">return</span> <span class="o">-</span><span class="mi">1</span> <span class="c1"># 未找到目标元素,返回 -1</span>
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<a id="__codelineno-2-15" name="__codelineno-2-15" href="#__codelineno-2-15"></a> <span class="k">return</span> <span class="n">i</span>
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<h1 id="103">10.3. 哈希优化策略<a class="headerlink" href="#103" title="Permanent link">¶</a></h1>
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<p>在算法题中,<strong>我们常通过将线性查找替换为哈希查找来降低算法的时间复杂度</strong>。我们借助一个算法题来加深理解。</p>
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<div class="admonition question">
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<p class="admonition-title">两数之和</p>
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<p>给定一个整数数组 <code>nums</code> 和一个整数目标值 <code>target</code> ,请在数组中搜索“和”为目标值 <code>target</code> 的两个整数,并返回他们在数组中的索引。注意,数组中同一个元素在答案里不能重复出现。返回任意一个解即可。</p>
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<p class="admonition-title">Question</p>
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<p>给定一个整数数组 <code>nums</code> 和一个目标元素 <code>target</code> ,请在数组中搜索“和”为 <code>target</code> 的两个元素,并返回它们的数组索引。返回任意一个解即可。</p>
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</div>
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<h2 id="1031">10.3.1. 线性查找:以时间换空间<a class="headerlink" href="#1031" title="Permanent link">¶</a></h2>
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<p>考虑直接遍历所有可能的组合。开启一个两层循环,在每轮中判断两个整数的和是否为 <code>target</code> ,若是,则返回它们的索引。</p>
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