LouisLan的学习简记 👋🏼

题目地址 https://leetcode.com/problems/valid-palindrome/ 题目描述 Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases. For example, "A man, a plan, a canal: Panama" is a palindrome. "race a car" is not a palindrome. Note: Have you consider that the string might be empty? This is a good question to ask during an interview. For the purpose of this problem, we define empty string as valid palindrome. 思路 验证回文字符串是比较常见的问题，所谓回文，就是一个正读和反读都一样的字符串，比如“level”....

题目地址 https://leetcode.com/problems/binary-tree-maximum-path-sum/ 题目描述 Given a non-empty binary tree, find the maximum path sum. For this problem, a path is defined as any sequence of nodes from some starting node to any node in the tree along the parent-child connections. The path must contain at least one node and does not need to go through the root. Example 1: Input: [1,2,3] 1 / \ 2 3 Output: 6 Example 2: Input: [-10,9,20,null,null,15,7] -10 / \ 9 20 / \ 15 7 Output: 42 思路 这道求二叉树的最大路径和是一道蛮有....

题目地址 https://leetcode.com/problems/best-time-to-buy-and-sell-stock-iii/ 题目描述 Say you have an array for which the i th element is the price of a given stock on day i. Design an algorithm to find the maximum profit. You may complete at most two transactions. Note: You may not engage in multiple transactions at the same time (i.e., you must sell the stock before you buy again). Example 1: Input: [3,3,5,0,0,3,1,4] Output: 6 Explanation: Buy on day 4 (price = 0) and sell on day 6 (price = 3), profit ....

题目地址 https://leetcode.com/problems/best-time-to-buy-and-sell-stock-ii/ 题目描述 Say you have an array for which the ith element is the price of a given stock on day i. Design an algorithm to find the maximum profit. You may complete as many transactions as you like (i.e., buy one and sell one share of the stock multiple times). Note: You may not engage in multiple transactions at the same time (i.e., you must sell the stock before you buy again). Example 1: Input: [7,1,5,3,6,4] Output: 7 Explanation....

题目地址 https://leetcode.com/problems/best-time-to-buy-and-sell-stock/ 题目描述 Say you have an array for which the ith element is the price of a given stock on day i. If you were only permitted to complete at most one transaction (i.e., buy one and sell one share of the stock), design an algorithm to find the maximum profit. Note that you cannot sell a stock before you buy one. Example 1: Input: [7,1,5,3,6,4] Output: 5 Explanation: Buy on day 2 (price = 1) and sell on day 5 (price = 6), profit = 6-1 =....

题目地址 https://leetcode.com/problems/triangle/ 题目描述 Given a triangle, find the minimum path sum from top to bottom. Each step you may move to adjacent numbers on the row below. For example, given the following triangle [ [2], [3,4], [6,5,7], [4,1,8,3] ] The minimum path sum from top to bottom is 11 (i.e., 2 + 3 + 5 + 1 = 11). Note: Bonus point if you are able to do this using only O ( n ) extra space, where n is the total number of rows in the triangle. 思路 这道题给了我们一个二维数组组成的三角形，让我们寻找一条自上而下的路径，使得路径和....

题目地址 https://leetcode.com/problems/pascals-triangle-ii/ 题目描述 Given a non-negative index k where k ≤ 33, return the k th index row of the Pascal's triangle. Note that the row index starts from 0. In Pascal's triangle, each number is the sum of the two numbers directly above it. Example: Input: 3 Output: [1,3,3,1] Follow up: Could you optimize your algorithm to use only O ( k ) extra space? 思路 杨辉三角想必大家并不陌生，应该最早出现在初高中的数学中，其实就是二项式系数的一种写法。 　　　　　　　　１ 　　　　　　　１　１ 　　　　　　１　２　１ 　　　　　１　３　３　１ 　　　　１　４　６　４　１....

题目地址 https://leetcode.com/problems/pascals-triangle/ 题目描述 Given a non-negative integer numRows , generate the first numRows of Pascal's triangle. In Pascal's triangle, each number is the sum of the two numbers directly above it. Example: Input: 5 Output: [ [1], [1,1], [1,2,1], [1,3,3,1], [1,4,6,4,1] ] 思路 杨辉三角是二项式系数的一种写法，如果熟悉杨辉三角的五个性质，那么很好生成，可参见我的上一篇博文Pascal's Triangle II。具体生成算是：每一行的首个和结尾一个数字都是 1，从第三行开始，中间的每个数字都是上一行的左右两个数字之和。 关键点解析 代码 class Solution { public: vector<vector<int>> gen....

题目地址 https://leetcode.com/problems/populating-next-right-pointers-in-each-node-ii/ 题目描述 Given a binary tree struct Node { int val; Node *left; Node *right; Node *next; } Populate each next pointer to point to its next right node. If there is no next right node, the next pointer should be set to NULL. Initially, all next pointers are set to NULL. Example: Input: root = [1,2,3,4,5,null,7] Output: [1,#,2,3,#,4,5,7,#] Explanation: Given the above binary tree (Figure A), your function should popula....

题目地址 https://leetcode.com/problems/populating-next-right-pointers-in-each-node/ 题目描述 You are given a perfect binary tree where all leaves are on the same level, and every parent has two children. The binary tree has the following definition: struct Node { int val; Node *left; Node *right; Node *next; } Populate each next pointer to point to its next right node. If there is no next right node, the next pointer should be set to NULL. Initially, all next pointers are set to NULL. Follow up: You m....