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<link rel="prev" href="../fractional_knapsack_problem/">
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15.3. 最大容量问题
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目录
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第一步:问题分析
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第二步:贪心策略确定
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代码实现
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第三步:正确性证明
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15.4. 最大切分乘积问题
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第一步:问题分析
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第二步:贪心策略确定
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代码实现
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第三步:正确性证明
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<p><img alt="最大容量问题的示例数据" src="../max_capacity_problem.assets/max_capacity_example.png" /></p>
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<p align="center"> Fig. 最大容量问题的示例数据 </p>
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<p><strong>第一步:问题分析</strong></p>
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<h3 id="_1">第一步:问题分析<a class="headerlink" href="#_1" title="Permanent link">¶</a></h3>
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<p>容器由任意两个隔板围成,<strong>因此本题的状态为两个隔板的索引,记为 <span class="arithmatex">\([i, j]\)</span></strong> 。</p>
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<p>根据定义,容量等于高度乘以宽度,其中高度由短板决定,宽度是两隔板的索引之差。设容量为 <span class="arithmatex">\(cap[i, j]\)</span> ,可得计算公式:</p>
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<div class="arithmatex">\[
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cap[i, j] = \min(ht[i], ht[j]) \times (j - i)
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\]</div>
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<p>设数组长度为 <span class="arithmatex">\(n\)</span> ,两个隔板的组合数量(即状态总数)为 <span class="arithmatex">\(C_n^2 = \frac{n(n - 1)}{2}\)</span> 个。最直接地,<strong>我们可以穷举所有状态</strong>,从而求得最大容量,时间复杂度为 <span class="arithmatex">\(O(n^2)\)</span> 。</p>
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<p><strong>第二步:贪心策略确定</strong></p>
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<h3 id="_2">第二步:贪心策略确定<a class="headerlink" href="#_2" title="Permanent link">¶</a></h3>
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<p>当然,这道题还有更高效率的解法。如下图所示,现选取一个状态 <span class="arithmatex">\([i, j]\)</span> ,其满足索引 <span class="arithmatex">\(i < j\)</span> 且高度 <span class="arithmatex">\(ht[i] < ht[j]\)</span> ,即 <span class="arithmatex">\(i\)</span> 为短板、 <span class="arithmatex">\(j\)</span> 为长板。</p>
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<p><img alt="初始状态" src="../max_capacity_problem.assets/max_capacity_initial_state.png" /></p>
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<p align="center"> Fig. 初始状态 </p>
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<p>代码实现如下所示。最多循环 <span class="arithmatex">\(n\)</span> 轮,<strong>因此时间复杂度为 <span class="arithmatex">\(O(n)\)</span></strong> 。变量 <span class="arithmatex">\(i\)</span> , <span class="arithmatex">\(j\)</span> , <span class="arithmatex">\(res\)</span> 使用常数大小额外空间,<strong>因此空间复杂度为 <span class="arithmatex">\(O(1)\)</span></strong> 。</p>
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<h3 id="_3">代码实现<a class="headerlink" href="#_3" title="Permanent link">¶</a></h3>
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<p>如下代码所示,循环最多 <span class="arithmatex">\(n\)</span> 轮,<strong>因此时间复杂度为 <span class="arithmatex">\(O(n)\)</span></strong> 。变量 <span class="arithmatex">\(i\)</span> , <span class="arithmatex">\(j\)</span> , <span class="arithmatex">\(res\)</span> 使用常数大小额外空间,<strong>因此空间复杂度为 <span class="arithmatex">\(O(1)\)</span></strong> 。</p>
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<p><strong>第三步:正确性证明</strong></p>
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<h3 id="_4">第三步:正确性证明<a class="headerlink" href="#_4" title="Permanent link">¶</a></h3>
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<p>之所以贪心比穷举更快,是因为每轮的贪心选择都会“跳过”一些状态。</p>
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<p>比如在状态 <span class="arithmatex">\(cap[i, j]\)</span> 下,<span class="arithmatex">\(i\)</span> 为短板、<span class="arithmatex">\(j\)</span> 为长板。若贪心地将短板 <span class="arithmatex">\(i\)</span> 向内移动一格,会导致以下状态被“跳过”,<strong>意味着之后无法验证这些状态的容量大小</strong>。</p>
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<div class="arithmatex">\[
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<a href="../../chapter_appendix/" class="md-footer__link md-footer__link--next" aria-label="下一页: 16. &nbsp; 附录" rel="next">
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16. 附录
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15.4. 最大切分乘积问题
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