题意:给出一棵无根树,每个节点有一个权值,现在要让dfs序的前k个结点的最小值最大,求出这个值. 考虑二分答案,把>=答案的点标记为1,<答案的点标记为0,现在的任务时使得dfs序的前k个节点都为1. 考虑树形DP. 用dp[u]表示从节点u开始在子树中进行dfs最多可以经过多少个为1的结点,显然,若某一个子树中节点全为1,那么这个可以加到dp[u]中,此外还可以在不全为1的子树中挑选一个加到dp[u]上. 那么答案就是从标记为1的节点当做根,选两颗不完全子树和所有的完全子树(包括从父亲向上的…

传送门 题目大意 给出一棵无根树,每个节点有一个权值,现在要让dfs序的前k个结点的最小值最大,求出这个值.分析 首先可以对这个值v进行二分然后01分数规划现在问题转化为求出一个dfs序,使得dfs序中的至少有k个1,这一步可以用树形dp来做.用dp[u]表示从节点u开始在子树中进行dfs最多可以经过多少个为1的结点,显然,若某一个子树中节点全为1,那么这个可以加到dp[u]中,此外还可以在不全为1的子树中挑选一个加到dp[u]上.现在考虑我们要求的答案,对于当前的结点u,那么答案为两棵不完全子…

One way to serialize a binary tree is to use pre-oder traversal. When we encounter a non-null node, we record the node's value. If it is a null node, we record using a sentinel value such as #. _9_ / \ 3 2 / \ / \ 4 1 # 6 / \ / \ / \ # # # # # # For…

Given an array of numbers, verify whether it is the correct preorder traversal sequence of a binary search tree. You may assume each number in the sequence is unique. Follow up: Could you do it using only constant space complexity? 这道题让给了我们一个一维数组,让我们…

Given a binary tree, return the preorder traversal of its nodes' values. For example:Given binary tree {1,#,2,3}, 1 \ 2 / 3 return [1,2,3]. Note: Recursive solution is trivial, could you do it iteratively? 一般我们提到树的遍历,最常见的有先序遍历,中序遍历,后序遍历和层序遍历,它们用递归实现起…

Given preorder and inorder traversal of a tree, construct the binary tree. Note:You may assume that duplicates do not exist in the tree. 这道题要求用先序和中序遍历来建立二叉树,跟之前那道Construct Binary Tree from Inorder and Postorder Traversal 由中序和后序遍历建立二叉树原理基本相同,针对这道题,由于先…

1. Description One way to serialize a binary tree is to use pre-order traversal. When we encounter a non-null node, we record the node's value. If it is a null node, we record using a sentinel value such as #. _9_ / \ 3 2 / \ / \ 4 1 # 6 / \ / \ / \…

验证一个list是不是一个BST的preorder traversal sequence. Given an array of numbers, verify whether it is the correct preorder traversal sequence of a binary search tree. You may assume each number in the sequence is unique. Follow up:Could you do it using only…

原题链接在这里:https://leetcode.com/problems/verify-preorder-sequence-in-binary-search-tree/ 题目: Given an array of numbers, verify whether it is the correct preorder traversal sequence of a binary search tree. You may assume each number in the sequence is u…

Problem Link: https://oj.leetcode.com/problems/construct-binary-tree-from-preorder-and-inorder-traversal/ The basic idea is same to that for Construct Binary Tree from Inorder and Postorder Traversal. We solve it using a recursive function. First, we…

Given a binary tree, return the preorder traversal of its nodes' values. For example:Given binary tree {1,#,2,3}, 1 \ 2 / 3 return [1,2,3]. /** * Definition for a binary tree node. * struct TreeNode { * int val; * TreeNode *left; * TreeNode *right; *…

Construct Binary Tree from Preorder and Inorder Traversal Given preorder and inorder traversal of a tree, construct the binary tree. 分析: 根据前序遍历和中序遍历构造一棵树,递归求解即可 /** * Definition for a binary tree node. * struct TreeNode { * int val; * TreeNode *left;…

Verify Preorder Sequence in Binary Search Tree \Given an array of numbers, verify whether it is the correct preorder traversal sequence of a binary search tree. You may assume each number in the sequence is unique. Follow up: Could you do it using on…

Given a binary tree, return the preorder traversal of its nodes' values. For example:Given binary tree {1,#,2,3}, 1 \ 2 / 3 return [1,2,3]. 二叉树的先序遍历.递归算法太简单了,重点讨论一下非递归的. 我自己写的时候一直纠结于何时弹出结点.后来看了几个版本的,发现可以跳过这个部分. 贴上我最喜欢的版本,逻辑最清楚 //我最喜欢的版本 逻辑清晰 vector<i…

Given a binary tree, return the preorder traversal of its nodes' values. For example:Given binary tree {1,#,2,3}, 1 \ 2 / 3 return [1,2,3]. Note: Recursive solution is trivial, could you do it iteratively? 这题简单,属于课本习题的难度,题目提示说用迭代试试,当然用递归也可以AC的.迭代的时候要…

Given preorder and inorder traversal of a tree, construct the binary tree. Note:You may assume that duplicates do not exist in the tree. 此题目有两种解决思路: 1)递归解决(比较好想)按照手动模拟的思路即可 2)非递归解决,用stack模拟递归 class Solution { public: TreeNode *buildTree(vector<int>&…

原题链接在这里:https://leetcode.com/problems/verify-preorder-serialization-of-a-binary-tree/ 题目: One way to serialize a binary tree is to use pre-order traversal. When we encounter a non-null node, we record the node's value. If it is a null node, we record…

题目: Given preorder and inorder traversal of a tree, construct the binary tree. Note:You may assume that duplicates do not exist in the tree. 解题思路分析: 前序遍历首先遍历根节点,然后依次遍历左右子树 中序遍历首先遍历左子树,然后遍历根结点,最后遍历右子树 根据二者的特点,前序遍历的首节点就是根结点,然后在中序序列中找出该结点位置(index),则该结点之…

Given preorder and inorder traversal of a tree, construct the binary tree. Note:You may assume that duplicates do not exist in the tree. Solution: Just do it recursively. TreeNode *build(vector<int> &preorder, int pstart, int pend, vector<int…

Binary Tree Preorder Traversal Given a binary tree, return the preorder traversal of its nodes' values. For example: Given binary tree {1,#,2,3}, 1 \ 2 / 3 return [1,2,3]. Note: Recursive solution is trivial, could you do it iteratively? Solution: 递归…

Given a binary tree, return the preorder traversal of its nodes' values. Given: 1 / \ 2 3 / \ 4 5 return [1,2,4,5,3]. Thinking: For this problem, you need to think about using recursive or non-recursive methods. As recursive method, we should think t…

详见:剑指 Offer 题目汇总索引:第6题 Binary Tree Postorder Traversal            Given a binary tree, return the postorder traversal of its nodes' values. For example: Given binary tree {1,#,2,3}, 1 \ 2 / 3 return [3,2,1]. Note: Recursive solution is trivial, could…

Construct Binary Tree from Inorder and Postorder Traversal OJ: https://oj.leetcode.com/problems/construct-binary-tree-from-inorder-and-postorder-traversal/ Given inorder and postorder traversal of a tree, construct the binary tree. Note: You may assu…

One way to serialize a binary tree is to use pre-order traversal. When we encounter a non-null node, we record the node's value. If it is a null node, we record using a sentinel value such as #. _9_ / \ 3 2 / \ / \ 4 1 # 6 / \ / \ / \ # # # # # # For…

LeetCode:Construct Binary Tree from Inorder and Postorder Traversal Given inorder and postorder traversal of a tree, construct the binary tree. Note:You may assume that duplicates do not exist in the tree.                                            …

前中后遍历 递归版 /* Recursive solution */ class Solution { public: vector<int> preorderTraversal(TreeNode *root) { vector<int> result; preorderTraversal(root, result); return result; } void preorderTraversal(TreeNode *root, vector<int>& res…

One way to serialize a binary tree is to use pre-order traversal. When we encounter a non-null node, we record the node's value. If it is a null node, we record using a sentinel value such as #. _9_ / \ 3 2 / \ / \ 4 1 # 6 / \ / \ / \ # # # # # # For…

Given a binary tree, return the preorder traversal of its nodes' values. For example: Given binary tree {1,#,2,3}, 1 \ 2 / 3 return [1,2,3]. 二叉树的前序遍历,根节点→左子树→右子树 解题思路一: 递归实现,JAVA实现如下: public List<Integer> preorderTraversal(TreeNode root) { List<I…

求问下各位大神,怎么判断一个按照Preorder traversal serialized的binary tree的序列是否正确呢?不能deserialize成树比如 A) 9 3 4 # # 1 # # 2 # 6 # #是对的,因为表示 9 / \ 3 2 / \ \ 4 1 6 B ) 9 3 4 # # 1 # #就是错的,因为无法反构造回一棵树 我觉得可以在字符串里找"n##"这种结构(对应tree里两个children都是Null的叶节点),找到之后就把"n##&…

[Inorder Traversal] Given a binary tree, return the inorder traversal of its nodes' values. For example:Given binary tree {1,#,2,3}, 1 \ 2 / 3 return [1,3,2]. Note: Recursive solution is trivial, could you do it iteratively? confused what "{1,#,2,3}&…