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<title>13.3.   DP 解题思路New - Hello 算法</title>
<title>14.3.   DP 解题思路New - Hello 算法</title>
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<div data-md-component="skip">
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跳转至
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<div class="md-header__topic" data-md-component="header-topic">
<span class="md-ellipsis">
13.3. &nbsp; DP 解题思路New
14.3. &nbsp; DP 解题思路New
</span>
</div>
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<div class="md-nav__link md-nav__link--index ">
<a href="../../chapter_divide_and_conquer/">12. &nbsp; &nbsp; 分治</a>
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12. &nbsp; &nbsp; 分治
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<a href="../../chapter_divide_and_conquer/divide_and_conquer/" class="md-nav__link">
12.1. &nbsp; 分治算法New
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12.2. &nbsp; 构建树问题New
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<a href="../../chapter_backtracking/">12. &nbsp; &nbsp; 回溯</a>
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12. &nbsp; &nbsp; 回溯
13. &nbsp; &nbsp; 回溯
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12.1. &nbsp; 回溯算法
13.1. &nbsp; 回溯算法
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@ -1820,7 +1901,7 @@
<li class="md-nav__item">
<a href="../../chapter_backtracking/permutations_problem/" class="md-nav__link">
12.2. &nbsp; 全排列问题
13.2. &nbsp; 全排列问题
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<li class="md-nav__item">
<a href="../../chapter_backtracking/subset_sum_problem/" class="md-nav__link">
12.3. &nbsp; 子集和问题
13.3. &nbsp; 子集和问题
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<li class="md-nav__item">
<a href="../../chapter_backtracking/n_queens_problem/" class="md-nav__link">
12.4. &nbsp; N 皇后问题
13.4. &nbsp; N 皇后问题
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</li>
@ -1862,7 +1943,7 @@
<li class="md-nav__item">
<a href="../../chapter_backtracking/summary/" class="md-nav__link">
12.5. &nbsp; 小结
13.5. &nbsp; 小结
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<div class="md-nav__link md-nav__link--index ">
<a href="../">13. &nbsp; &nbsp; 动态规划</a>
<a href="../">14. &nbsp; &nbsp; 动态规划</a>
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13. &nbsp; &nbsp; 动态规划
14. &nbsp; &nbsp; 动态规划
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<li class="md-nav__item">
<a href="../intro_to_dynamic_programming/" class="md-nav__link">
13.1. &nbsp; 初探动态规划New
14.1. &nbsp; 初探动态规划New
</a>
</li>
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<li class="md-nav__item">
<a href="../dp_problem_features/" class="md-nav__link">
13.2. &nbsp; DP 问题特性New
14.2. &nbsp; DP 问题特性New
</a>
</li>
@ -1978,12 +2059,12 @@
<label class="md-nav__link md-nav__link--active" for="__toc">
13.3. &nbsp; DP 解题思路New
14.3. &nbsp; DP 解题思路New
<span class="md-nav__icon md-icon"></span>
</label>
<a href="./" class="md-nav__link md-nav__link--active">
13.3. &nbsp; DP 解题思路New
14.3. &nbsp; DP 解题思路New
</a>
@ -2002,36 +2083,36 @@
<ul class="md-nav__list" data-md-component="toc" data-md-scrollfix>
<li class="md-nav__item">
<a href="#1331" class="md-nav__link">
13.3.1. &nbsp; 问题判断
<a href="#1431" class="md-nav__link">
14.3.1. &nbsp; 问题判断
</a>
</li>
<li class="md-nav__item">
<a href="#1332" class="md-nav__link">
13.3.2. &nbsp; 问题求解
<a href="#1432" class="md-nav__link">
14.3.2. &nbsp; 问题求解
</a>
</li>
<li class="md-nav__item">
<a href="#1333" class="md-nav__link">
13.3.3. &nbsp; 方法一:暴力搜索
<a href="#1433" class="md-nav__link">
14.3.3. &nbsp; 方法一:暴力搜索
</a>
</li>
<li class="md-nav__item">
<a href="#1334" class="md-nav__link">
13.3.4. &nbsp; 方法二:记忆化搜索
<a href="#1434" class="md-nav__link">
14.3.4. &nbsp; 方法二:记忆化搜索
</a>
</li>
<li class="md-nav__item">
<a href="#1335" class="md-nav__link">
13.3.5. &nbsp; 方法三:动态规划
<a href="#1435" class="md-nav__link">
14.3.5. &nbsp; 方法三:动态规划
</a>
</li>
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<li class="md-nav__item">
<a href="../knapsack_problem/" class="md-nav__link">
13.4. &nbsp; 0-1 背包问题New
14.4. &nbsp; 0-1 背包问题New
</a>
</li>
@ -2066,7 +2147,7 @@
<li class="md-nav__item">
<a href="../unbounded_knapsack_problem/" class="md-nav__link">
13.5. &nbsp; 完全背包问题New
14.5. &nbsp; 完全背包问题New
</a>
</li>
@ -2080,7 +2161,7 @@
<li class="md-nav__item">
<a href="../edit_distance_problem/" class="md-nav__link">
13.6. &nbsp; 编辑距离问题New
14.6. &nbsp; 编辑距离问题New
</a>
</li>
@ -2094,76 +2175,7 @@
<li class="md-nav__item">
<a href="../summary/" class="md-nav__link">
13.7. &nbsp; 小结New
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14. &nbsp; &nbsp; 附录
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14.1. &nbsp; 编程环境安装
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14.2. &nbsp; 一起参与创作
14.7. &nbsp; 小结New
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15.1. &nbsp; 编程环境安装
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15.2. &nbsp; 一起参与创作
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@ -2206,8 +2287,8 @@
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参考文献
</label>
@ -2248,36 +2329,36 @@
<ul class="md-nav__list" data-md-component="toc" data-md-scrollfix>
<li class="md-nav__item">
<a href="#1331" class="md-nav__link">
13.3.1. &nbsp; 问题判断
<a href="#1431" class="md-nav__link">
14.3.1. &nbsp; 问题判断
</a>
</li>
<li class="md-nav__item">
<a href="#1332" class="md-nav__link">
13.3.2. &nbsp; 问题求解
<a href="#1432" class="md-nav__link">
14.3.2. &nbsp; 问题求解
</a>
</li>
<li class="md-nav__item">
<a href="#1333" class="md-nav__link">
13.3.3. &nbsp; 方法一:暴力搜索
<a href="#1433" class="md-nav__link">
14.3.3. &nbsp; 方法一:暴力搜索
</a>
</li>
<li class="md-nav__item">
<a href="#1334" class="md-nav__link">
13.3.4. &nbsp; 方法二:记忆化搜索
<a href="#1434" class="md-nav__link">
14.3.4. &nbsp; 方法二:记忆化搜索
</a>
</li>
<li class="md-nav__item">
<a href="#1335" class="md-nav__link">
13.3.5. &nbsp; 方法三:动态规划
<a href="#1435" class="md-nav__link">
14.3.5. &nbsp; 方法三:动态规划
</a>
</li>
@ -2305,13 +2386,13 @@
<h1 id="133">13.3. &nbsp; 动态规划解题思路<a class="headerlink" href="#133" title="Permanent link">&para;</a></h1>
<h1 id="143">14.3. &nbsp; 动态规划解题思路<a class="headerlink" href="#143" title="Permanent link">&para;</a></h1>
<p>上两节介绍了动态规划问题的主要特征,接下来我们一起探究两个更加实用的问题:</p>
<ol>
<li>如何判断一个问题是不是动态规划问题?</li>
<li>求解动态规划问题该从何处入手,完整步骤是什么?</li>
</ol>
<h2 id="1331">13.3.1. &nbsp; 问题判断<a class="headerlink" href="#1331" title="Permanent link">&para;</a></h2>
<h2 id="1431">14.3.1. &nbsp; 问题判断<a class="headerlink" href="#1431" title="Permanent link">&para;</a></h2>
<p>总的来说,如果一个问题包含重叠子问题、最优子结构,并满足无后效性,那么它通常就适合用动态规划求解,但我们很难从问题描述上直接提取出这些特性。因此我们通常会放宽条件,<strong>先观察问题是否适合使用回溯(穷举)解决</strong></p>
<p><strong>适合用回溯解决的问题通常满足“决策树模型”</strong>,这种问题可以使用树形结构来描述,其中每一个节点代表一个决策,每一条路径代表一个决策序列。</p>
<p>换句话说,如果问题包含明确的决策概念,并且解是通过一系列决策产生的,那么它就满足决策树模型,通常可以使用回溯来解决。</p>
@ -2326,7 +2407,7 @@
<li>问题描述中有明显的排列组合的特征,需要返回具体的多个方案。</li>
</ul>
<p>如果一个问题满足决策树模型,并具有较为明显的“加分项“,我们就可以假设它是一个动态规划问题,并尝试求解它。</p>
<h2 id="1332">13.3.2. &nbsp; 问题求解<a class="headerlink" href="#1332" title="Permanent link">&para;</a></h2>
<h2 id="1432">14.3.2. &nbsp; 问题求解<a class="headerlink" href="#1432" title="Permanent link">&para;</a></h2>
<p>动态规划的解题流程可能会因问题的性质和难度而有所不同,但通常遵循以下步骤:描述决策,定义状态,建立 <span class="arithmatex">\(dp\)</span> 表,推导状态转移方程,确定边界条件等。</p>
<p>为了更形象地展示解题步骤,我们使用一个经典问题「最小路径和」来举例。</p>
<div class="admonition question">
@ -2374,7 +2455,7 @@ dp[i, j] = \min(dp[i-1, j], dp[i, j-1]) + grid[i, j]
<p>边界条件即初始状态,在搜索中用于剪枝,在动态规划中用于初始化 <span class="arithmatex">\(dp\)</span> 表。状态转移顺序的核心是要保证在计算当前问题时,所有它依赖的更小子问题都已经被正确地计算出来。</p>
</div>
<p>接下来,我们就可以实现动态规划代码了。然而,由于子问题分解是一种从顶至底的思想,因此按照“暴力搜索 <span class="arithmatex">\(\rightarrow\)</span> 记忆化搜索 <span class="arithmatex">\(\rightarrow\)</span> 动态规划”的顺序实现更加符合思维习惯。</p>
<h2 id="1333">13.3.3. &nbsp; 方法一:暴力搜索<a class="headerlink" href="#1333" title="Permanent link">&para;</a></h2>
<h2 id="1433">14.3.3. &nbsp; 方法一:暴力搜索<a class="headerlink" href="#1433" title="Permanent link">&para;</a></h2>
<p>从状态 <span class="arithmatex">\([i, j]\)</span> 开始搜索,不断分解为更小的状态 <span class="arithmatex">\([i-1, j]\)</span><span class="arithmatex">\([i, j-1]\)</span> ,包括以下递归要素:</p>
<ul>
<li><strong>递归参数</strong>:状态 <span class="arithmatex">\([i, j]\)</span> <strong>返回值</strong>:从 <span class="arithmatex">\([0, 0]\)</span><span class="arithmatex">\([i, j]\)</span> 的最小路径和 <span class="arithmatex">\(dp[i, j]\)</span> </li>
@ -2492,7 +2573,7 @@ dp[i, j] = \min(dp[i-1, j], dp[i, j-1]) + grid[i, j]
<p align="center"> Fig. 暴力搜索递归树 </p>
<p>每个状态都有向下和向右两种选择,从左上角走到右下角总共需要 <span class="arithmatex">\(m + n - 2\)</span> 步,所以最差时间复杂度为 <span class="arithmatex">\(O(2^{m + n})\)</span> 。请注意,这种计算方式未考虑临近网格边界的情况,当到达网络边界时只剩下一种选择。因此实际的路径数量会少一些。</p>
<h2 id="1334">13.3.4. &nbsp; 方法二:记忆化搜索<a class="headerlink" href="#1334" title="Permanent link">&para;</a></h2>
<h2 id="1434">14.3.4. &nbsp; 方法二:记忆化搜索<a class="headerlink" href="#1434" title="Permanent link">&para;</a></h2>
<p>为了避免重复计算重叠子问题,我们引入一个和网格 <code>grid</code> 相同尺寸的记忆列表 <code>mem</code> ,用于记录各个子问题的解,提升搜索效率。</p>
<div class="tabbed-set tabbed-alternate" data-tabs="2:11"><input checked="checked" id="__tabbed_2_1" name="__tabbed_2" type="radio" /><input id="__tabbed_2_2" name="__tabbed_2" type="radio" /><input id="__tabbed_2_3" name="__tabbed_2" type="radio" /><input id="__tabbed_2_4" name="__tabbed_2" type="radio" /><input id="__tabbed_2_5" name="__tabbed_2" type="radio" /><input id="__tabbed_2_6" name="__tabbed_2" type="radio" /><input id="__tabbed_2_7" name="__tabbed_2" type="radio" /><input id="__tabbed_2_8" name="__tabbed_2" type="radio" /><input id="__tabbed_2_9" name="__tabbed_2" type="radio" /><input id="__tabbed_2_10" name="__tabbed_2" type="radio" /><input id="__tabbed_2_11" name="__tabbed_2" type="radio" /><div class="tabbed-labels"><label for="__tabbed_2_1">Java</label><label for="__tabbed_2_2">C++</label><label for="__tabbed_2_3">Python</label><label for="__tabbed_2_4">Go</label><label for="__tabbed_2_5">JavaScript</label><label for="__tabbed_2_6">TypeScript</label><label for="__tabbed_2_7">C</label><label for="__tabbed_2_8">C#</label><label for="__tabbed_2_9">Swift</label><label for="__tabbed_2_10">Zig</label><label for="__tabbed_2_11">Dart</label></div>
<div class="tabbed-content">
@ -2624,7 +2705,7 @@ dp[i, j] = \min(dp[i-1, j], dp[i, j-1]) + grid[i, j]
<p><img alt="记忆化搜索递归树" src="../dp_solution_pipeline.assets/min_path_sum_dfs_mem.png" /></p>
<p align="center"> Fig. 记忆化搜索递归树 </p>
<h2 id="1335">13.3.5. &nbsp; 方法三:动态规划<a class="headerlink" href="#1335" title="Permanent link">&para;</a></h2>
<h2 id="1435">14.3.5. &nbsp; 方法三:动态规划<a class="headerlink" href="#1435" title="Permanent link">&para;</a></h2>
<p>动态规划代码是从底至顶的,仅需循环即可实现。</p>
<div class="tabbed-set tabbed-alternate" data-tabs="3:11"><input checked="checked" id="__tabbed_3_1" name="__tabbed_3" type="radio" /><input id="__tabbed_3_2" name="__tabbed_3" type="radio" /><input id="__tabbed_3_3" name="__tabbed_3" type="radio" /><input id="__tabbed_3_4" name="__tabbed_3" type="radio" /><input id="__tabbed_3_5" name="__tabbed_3" type="radio" /><input id="__tabbed_3_6" name="__tabbed_3" type="radio" /><input id="__tabbed_3_7" name="__tabbed_3" type="radio" /><input id="__tabbed_3_8" name="__tabbed_3" type="radio" /><input id="__tabbed_3_9" name="__tabbed_3" type="radio" /><input id="__tabbed_3_10" name="__tabbed_3" type="radio" /><input id="__tabbed_3_11" name="__tabbed_3" type="radio" /><div class="tabbed-labels"><label for="__tabbed_3_1">Java</label><label for="__tabbed_3_2">C++</label><label for="__tabbed_3_3">Python</label><label for="__tabbed_3_4">Go</label><label for="__tabbed_3_5">JavaScript</label><label for="__tabbed_3_6">TypeScript</label><label for="__tabbed_3_7">C</label><label for="__tabbed_3_8">C#</label><label for="__tabbed_3_9">Swift</label><label for="__tabbed_3_10">Zig</label><label for="__tabbed_3_11">Dart</label></div>
<div class="tabbed-content">
@ -2997,7 +3078,7 @@ dp[i, j] = \min(dp[i-1, j], dp[i, j-1]) + grid[i, j]
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