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Tree/README.md
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| # 树的总结以及 Java 代码模板 | |||
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将来还会有其他作者添加其它语言的模板，全文不一定限定在 Java 代码模板。
标题建议用「数据结构」+「算法思想」，例如「树与递归」或者「树与分治」。
Tree/README.md
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| 树的示例图 | ||
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| ### 树的结点： |
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以 # 开头的行，后面就不用加 : 了，建议全文检查一下。
Tree/README.md
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| return res; | ||
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| private void traversalHelper(List<Integer> res, TreeNode root) { |
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List<Integer> res 这个参数放在第 2 个参数会好一些（建议）。
| ## 学习内容： | ||
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| ### 树的遍历（Traversal） ： | ||
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可以在介绍遍历的时候，先介绍两种遍历的思想：「深度优先遍历」和「广度优先遍历」（这里可以配图，这篇 题解 里的图可以放在这里）。
「深度优先遍历」每一个结点会访问 3 次（这个提法待讨论，可能会让读者迷惑），在不同的顺序执行不同的操作，就有「前序」「中序」「后序」之分。
「广度优先遍历」以一种类似水波扩散的方式遍历所有结点。
这两种遍历的思想相当重要，需要在这个地方让读者建立起形象的思维。然后可以引入完成这两种遍历需要借助的数据结构「栈」和「队列」。
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| 后序遍历的顺序，先遍历左子树，再遍历右子树，最后把根结点加入访问序列。[LeetCode 145](https://leetcode-cn.com/problems/binary-tree-postorder-traversal/) | ||
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| 后序遍历的题目非常多，而且难度较高的通常是后序遍历的题，但是只要能够清晰地抓住解题重点，这些题目都能够迎刃而解。做一个形象的比喻，把这个棵树比成一个学校，根结点就像是校长，它的子结点就是各个学院的院长，院长的子结点就是系主任，系主任的子结点是班长，班长的子结点就是班里同学，每个同学就是一个叶结点。如果这时有一个任务，让校长统计学校人数，校长会一个一个人数吗？当然不会，他会让院长去统计每个院的人数，统计完结果之后，自己要做的就是把每个院人数加起来，再加上自己一个人，就是学校总人数了。每个院长，系主任，也都是一样，把任务传递下去，他们做的事情和校长一样，都是统计一下结果，再加上自己。 |
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这个例子很好，很形象地说明了后序遍历的应用，在选择例题的时候，可以标注出来哪些是后序遍历思想的应用。
如果可以的话，配一个图可能会让读者印象更深。
Tree/README.md
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| // 是的话，把他加入统计结果 | ||
| res += root.left.val; | ||
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| 前序遍历的顺序，先遍历根结点，再遍历左子树，最后遍历右子树。[LeetCode 102](https://leetcode-cn.com/problems/binary-tree-level-order-traversal/) | ||
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| 层序遍历的根据到根结点的距离，逐层从左往右遍历，需要用到 BFS 。关于 BFS 就不在这里进行介绍了。 |
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层序遍历和 BFS 是一回事，这里需要重新组织一下语言。层序遍历其实最直观，是可以一句两句说清楚的。
Tree/README.md
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| if (root == null) { | ||
| return res; | ||
| } | ||
| Queue<TreeNode> q = new LinkedList<>(); |
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