From 9eea30cb9f4e9811aaf3aa8f087ac640fc703727 Mon Sep 17 00:00:00 2001 From: youngyangyang04 <826123027@qq.com> Date: Tue, 6 Oct 2020 10:38:33 +0800 Subject: [PATCH] Update --- README.md | 1 + pics/404.左叶子之和1.png | Bin 0 -> 22375 bytes problems/0100.相同的树.md | 30 +++--- problems/0257.二叉树的所有路径.md | 4 + problems/0404.左叶子之和.md | 114 +++++++++++++++++---- problems/二叉树中递归带着回溯.md | 72 +++++++++++++ 6 files changed, 185 insertions(+), 36 deletions(-) create mode 100644 pics/404.左叶子之和1.png create mode 100644 problems/二叉树中递归带着回溯.md diff --git a/README.md b/README.md index 3f432a75..18d36d58 100644 --- a/README.md +++ b/README.md @@ -94,6 +94,7 @@ * [二叉树:我有多少个节点?](https://mp.weixin.qq.com/s/2_eAjzw-D0va9y4RJgSmXw) * [二叉树:我平衡么?](https://mp.weixin.qq.com/s/isUS-0HDYknmC0Rr4R8mww) * [二叉树:找我的所有路径?](https://mp.weixin.qq.com/s/Osw4LQD2xVUnCJ-9jrYxJA) + * [还在玩耍的你,该总结啦!(本周小结之二叉树)](https://mp.weixin.qq.com/s/QMBUTYnoaNfsVHlUADEzKg) (持续更新中....) diff --git a/pics/404.左叶子之和1.png b/pics/404.左叶子之和1.png new 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zwqpnSy{(W9H_SToq%Kr%5rqj$4K;%40uhYZw=mh9otg`Fv&MVsVM;_|5cWSQ5ElcU ztOusjVI>6k>%=)}N*Yf_LiL>&@)OobmE|p5kEmOSPAW1bV~(pfyD+zG#d!^S#W>0NGC6sZ4xJUat zz{gkkJz|m+duz@#qKND)YZidgz%(_ZUz8HLXu73@G`zhxpdIxEh8zxkZGZ*2+9b7u zKW6Hw2p+YYa-3VSX}H6qHK~R*e?sAK7xSq~+i`<`Vu_r8>>akHaDp0Sp1@^wRQye5 z@cdAEpDhmJhaNITwUMt)6PzPH?5~oU0G>VdVy+d|`ukZ&l}rinNCtf$wj1ITRGakR z*am$zQJM%3Rx!b!*a_k8)JOc>E>Br-;R%kjrih;&Dn|#N56d2~*Z%t%STTPCJWlFe z=-ipk-_Otdk2y37aY{^>reSval != tree2->val) return false; // 注意这里我没有 代码如下: ``` -bool outside = compare(tree1->left, tree2->right); // 左子树:左、 右子树:左 -bool inside = compare(tree1->right, tree2->left); // 左子树:右、 右子树:右 +bool outside = compare(tree1->left, tree2->left); // 左子树:左、 右子树:左 +bool inside = compare(tree1->right, tree2->right); // 左子树:右、 右子树:右 bool isSame = outside && inside; // 左子树:中、 右子树:中(逻辑处理) return isSame; ``` --------------------------------------------------------------- 写到这 最后递归的C++整体代码如下: ``` class Solution { public: - bool compare(TreeNode* left, TreeNode* right) { - // 首先排除空节点的情况 - if (left == NULL && right != NULL) return false; - else if (left != NULL && right == NULL) return false; - else if (left == NULL && right == NULL) return true; - // 排除了空节点,再排除数值不相同的情况 - else if (left->val != right->val) return false; + bool compare(TreeNode* tree1, TreeNode* tree2) { + if (tree1 == NULL && tree2 != NULL) return false; + else if (tree1 != NULL && tree2 == NULL) return false; + else if (tree1 == NULL && tree2 == NULL) return true; + else if (tree1->val != tree2->val) return false; // 注意这里我没有使用else // 此时就是:左右节点都不为空,且数值相同的情况 // 此时才做递归,做下一层的判断 - bool outside = compare(left->left, right->right); // 左子树:左、 右子树:左 - bool inside = compare(left->right, right->left); // 左子树:右、 右子树:右 - bool isSame = outside && inside; // 左子树:中、 右子树:中 (逻辑处理) + bool outside = compare(tree1->left, tree2->left); // 左子树:左、 右子树:左 + bool inside = compare(tree1->right, tree2->right); // 左子树:右、 右子树:右 + bool isSame = outside && inside; // 左子树:中、 右子树:中(逻辑处理) return isSame; } - bool isSymmetric(TreeNode* root) { - if (root == NULL) return true; - return compare(root->left, root->right); + bool isSameTree(TreeNode* p, TreeNode* q) { + return compare(p, q); } }; ``` +-------------------------------------------------------------- 写到这 + **我给出的代码并不简洁,但是把每一步判断的逻辑都清楚的描绘出来了。** 如果上来就看网上各种简洁的代码,看起来真的很简单,但是很多逻辑都掩盖掉了,而题解可能也没有把掩盖掉的逻辑说清楚。 diff --git a/problems/0257.二叉树的所有路径.md b/problems/0257.二叉树的所有路径.md index 3fcbe67d..1e083c78 100644 --- a/problems/0257.二叉树的所有路径.md +++ b/problems/0257.二叉树的所有路径.md @@ -207,8 +207,12 @@ public: 那么在如上代码中,**貌似没有看到回溯的逻辑,其实不然,回溯就隐藏在`traversal(cur->left, path + "->", result);`中的 `path + "->"`。** 每次函数调用完,path依然是没有加上"->" 的,这就是回溯了。 + + **综合以上,第二种递归的代码虽然精简但把很多重要的点隐藏在了代码细节里,第一种递归写法虽然代码多一些,但是把每一个逻辑处理都完整的展现了出来了。** + + ## 迭代法 至于非递归的方式,我们可以依然可以使用前序遍历的迭代方式来模拟遍历路径的过程,对该迭代方式不了解的同学,可以看文章[二叉树:听说递归能做的,栈也能做!](https://mp.weixin.qq.com/s/c_zCrGHIVlBjUH_hJtghCg)和[二叉树:前中后序迭代方式的写法就不能统一一下么?](https://mp.weixin.qq.com/s/WKg0Ty1_3SZkztpHubZPRg)。 diff --git a/problems/0404.左叶子之和.md b/problems/0404.左叶子之和.md index d1a17f7b..4d8d5f04 100644 --- a/problems/0404.左叶子之和.md +++ b/problems/0404.左叶子之和.md @@ -1,9 +1,19 @@ ## 题目地址 https://leetcode-cn.com/problems/sum-of-left-leaves/ -## 思路 +> 有的题目就是 -首先要注意是判断左叶子呢,不是左子树,不要上来想着层序遍历。 +# 404.左叶子之和 + +计算给定二叉树的所有左叶子之和。 + +示例: + + + +# 思路 + +**首先要注意是判断左叶子,不是二叉树左侧节点,所以不要上来想着层序遍历。** 因为题目中其实没有说清楚左叶子究竟是什么节点,那么我来给出左叶子的明确定义:**如果左节点不为空,且左节点没有左右孩子,那么这个节点就是左叶子** @@ -13,42 +23,99 @@ https://leetcode-cn.com/problems/sum-of-left-leaves/ **其实是0,因为这棵树根本没有左叶子!** -那么判断左叶子,如果**判断当前节点是不是左叶子是无法判断的,必须要通过节点的父节点来判断其左孩子是不是左叶子。** +那么**判断当前节点是不是左叶子是无法判断的,必须要通过节点的父节点来判断其左孩子是不是左叶子。** -判断的根据如下: + +如果该节点的左节点不为空,该节点的左节点的左节点为空,该节点的左节点的右节点为空,则找到了一个左叶子,判断代码如下: ``` - if (node->left != NULL && node->left->left == NULL && node->left->right == NULL) { - sum = node->left->val; - } +if (node->left != NULL && node->left->left == NULL && node->left->right == NULL) { + 左叶子节点处理逻辑 +} ``` -递归写法代码如下: +## 递归法 -## 递归C++代码 +递归的遍历顺序为后序遍历(左右中),是因为要通过递归函数的返回值来累加求取左叶子数值之和。。 + +递归三部曲: + +1. 确定递归函数的参数和返回值 + +判断一个树的左叶子节点之和,那么一定要传入树的根节点,递归函数的返回值为数值之和,所以为int + +使用题目中给出的函数就可以了。 + +2. 确定终止条件 + +依然是 +``` +if (root == NULL) return 0; +``` + +3. 确定单层递归的逻辑 + +当遇到左叶子节点的时候,记录数值,然后通过递归求取左子树左叶子之和,和 右子树左叶子之和,相加便是整个树的左叶子之和。 + +代码如下: + +``` +int leftValue = sumOfLeftLeaves(root->left); // 左 +int rightValue = sumOfLeftLeaves(root->right); // 右 + // 中 +int midValue = 0; +if (root->left && !root->left->left && !root->left->right) { + midValue = root->left->val; +} +int sum = midValue + leftValue + rightValue; +return sum; + +``` + + +整体递归代码如下: ``` class Solution { public: int sumOfLeftLeaves(TreeNode* root) { if (root == NULL) return 0; - int sum = 0; - if (root->left != NULL && root->left->left == NULL && root->left->right == NULL) { - sum = root->left->val; + + int leftValue = sumOfLeftLeaves(root->left); // 左 + int rightValue = sumOfLeftLeaves(root->right); // 右 + // 中 + int midValue = 0; + if (root->left && !root->left->left && !root->left->right) { // 中 + midValue = root->left->val; } - return sum + sumOfLeftLeaves(root->left) + sumOfLeftLeaves(root->right); + int sum = midValue + leftValue + rightValue; + return sum; } }; ``` -递归的过程其实就是二叉树的前序遍历,那么写过二叉树的同学都知道,既然是二叉树的前序遍历,能写出递归,就能写出非递归。 +以上代码精简之后如下: -如果对二叉树的各种递归和非递归的写法不熟悉,可以看我的这个题解: -[彻底吃透二叉树的前中后序递归法和迭代法!!](https://leetcode-cn.com/problems/binary-tree-inorder-traversal/solution/che-di-chi-tou-er-cha-shu-de-qian-zhong-hou-xu-d-2/) +``` +class Solution { +public: + int sumOfLeftLeaves(TreeNode* root) { + if (root == NULL) return 0; + int midValue = 0; + if (root->left != NULL && root->left->left == NULL && root->left->right == NULL) { + midValue = root->left->val; + } + return midValue + sumOfLeftLeaves(root->left) + sumOfLeftLeaves(root->right); + } +}; +``` -那么非递归版本善良登场,判断条件都是一样的 +## 迭代法 -## 非递归C++代码 + +本题使用了后序遍历啊,那么参考文章 [二叉树:听说递归能做的,栈也能做!](https://mp.weixin.qq.com/s/c_zCrGHIVlBjUH_hJtghCg)和[二叉树:前中后序迭代方式的写法就不能统一一下么?](https://mp.weixin.qq.com/s/WKg0Ty1_3SZkztpHubZPRg)中的写法,同样可以写出一个后序遍历的迭代法。 + +判断条件都是一样的,代码如下: ``` @@ -70,8 +137,15 @@ public: } return result; } - - }; ``` +# 总结 + +这道题目要求左叶子之和,其实是比较绕的,因为不能判断本节点是不是左叶子节点。 + +此时就要通过节点的父节点来判断其左孩子是不是左叶子了。 + +**平时我们解二叉树的题目时,已经习惯了通过节点的左右孩子判断本节点的属性,而本题我们要通过节点的父节点判断本节点的属性。** + +希望通过这道题目,可以扩展大家对二叉树的解题思路。 diff --git a/problems/二叉树中递归带着回溯.md b/problems/二叉树中递归带着回溯.md new file mode 100644 index 00000000..3cb00f24 --- /dev/null +++ b/problems/二叉树中递归带着回溯.md @@ -0,0 +1,72 @@ + +在上一面 + + +``` +class Solution { +private: + void traversal(TreeNode* cur, string path, vector& result) { + path += to_string(cur->val); // 中 + if (cur->left == NULL && cur->right == NULL) { + result.push_back(path); + return; + } + + if (cur->left) { + path += "->"; + traversal(cur->left, path, result); // 左 + path.pop_back(); // 回溯 + path.pop_back(); + } + if (cur->right) { + path += "->"; + traversal(cur->right, path, result); // 右 + path.pop_back(); // 回溯 + path.pop_back(); + } + } + +public: + vector binaryTreePaths(TreeNode* root) { + vector result; + string path; + if (root == NULL) return result; + traversal(root, path, result); + return result; + + } +}; +``` + +没有回溯了 +``` +class Solution { +private: + void traversal(TreeNode* cur, string path, vector& result) { + path += to_string(cur->val); // 中 + if (cur->left == NULL && cur->right == NULL) { + result.push_back(path); + return; + } + + if (cur->left) { + path += "->"; + traversal(cur->left, path, result); // 左 + } + if (cur->right) { + path += "->"; + traversal(cur->right, path, result); // 右 + } + } + +public: + vector binaryTreePaths(TreeNode* root) { + vector result; + string path; + if (root == NULL) return result; + traversal(root, path, result); + return result; + + } +}; +```