Given a linked list, return the node where the cycle begins. If there is no cycle, return null. Follow up:Can you solve it without using extra space? 思路: 做过,就当复习了. 先用快慢指针判断相交,关键是环开始点的获取. 用上图说明一下,设非环的部分长度为a(包括环的入口点), 环的长度为b(包括环的入口点). 快慢指针相交的位置为绿色的点,距离…

Given a binary tree and a sum, determine if the tree has a root-to-leaf path such that adding up all the values along the path equals the given sum. For example:Given the below binary tree and sum = 22, 5 / \ 4 8 / / \ 11 13 4 / \ \ 7 2 1 return true…

Given a linked list, return the node where the cycle begins. If there is no cycle, return null. Follow up: Can you solve it without using extra space? 思路:由[Leetcode]Linked List Cycle可知.利用一快一慢两个指针可以推断出链表是否存在环路. 如果两个指针相遇之前slow走了s步,则fast走了2s步.而且fast已经在长…

Given a linked list, return the node where the cycle begins. If there is no cycle, return null. Follow up:Can you solve it without using extra space? 思路 这题是Linked List Cycle的进阶版 Given a linked list, determine if it has a cycle in it. bool hasCycle(Li…

这是LeetCode里的第142道题. 题目要求: 给定一个链表,返回链表开始入环的第一个节点. 如果链表无环,则返回 null. 说明:不允许修改给定的链表. 进阶:你是否可以不用额外空间解决此题? 起初我在做这道题的时候,以为挺简单的,以为循环链表都是已头节点为循环头,结果... ~~~~(>_<)~~~~ 没考虑到链中任一个节点都可能是循环头的头节点. 一开始比较贪心,就只设置的一个指针p来判断是否循环,结果思考不充分,没考虑到第二种特殊的情况,导致错了很多次. 然后经过多次测试后终于成…

Given a linked list, swap every two adjacent nodes and return its head. For example,Given 1->2->3->4, you should return the list as 2->1->4->3. Your algorithm should use only constant space. You may not modify the values in the list, onl…

Given a sorted array of integers, find the starting and ending position of a given target value. Your algorithm's runtime complexity must be in the order of O(log n). If the target is not found in the array, return [-1, -1]. For example,Given [5, 7,…

Implement an iterator over a binary search tree (BST). Your iterator will be initialized with the root node of a BST. Calling next() will return the next smallest number in the BST. Note: next() and hasNext() should run in average O(1) time and uses…

Given a binary tree, determine if it is a valid binary search tree (BST). Assume a BST is defined as follows: The left subtree of a node contains only nodes with keys less than the node's key. The right subtree of a node contains only nodes with keys…

Given an array of n positive integers and a positive integer s, find the minimal length of a subarray of which the sum ≥ s. If there isn't one, return 0 instead. For example, given the array [2,3,1,2,4,3] and s = 7,the subarray [4,3] has the minimal…

Evaluate the value of an arithmetic expression in Reverse Polish Notation. Valid operators are +, -, *, /. Each operand may be an integer or another expression. Some examples: ["2", "1", "+", "3", "*"] -&g…

Given n non-negative integers a1, a2, ..., an, where each represents a point at coordinate (i, ai). n vertical lines are drawn such that the two endpoints of line i is at (i, ai) and (i, 0). Find two lines, which together with x-axis forms a containe…

[LeetCode]120. Triangle 解题报告(Python) 作者: 负雪明烛 id: fuxuemingzhu 个人博客: http://fuxuemingzhu.cn/ 题目地址https://leetcode.com/problems/triangle/description/ 题目描述: Given a triangle, find the minimum path sum from top to bottom. Each step you may move to adjac…

Given a linked list, return the node where the cycle begins. If there is no cycle, return null. 解题: 寻找单链表中环的入口,如果不存在环则返回null. 假设单链表有环,先利用判断单链表是否有环(Linked List Cycle)的方法,找到快慢指针的交点. 假设环的入口在第K个结点处,那么此时快慢指针的交点,距离环入口,还差k个距离. 快慢指针相交后,在链表头位置同时启动另一个指针target,…

[题意] 给两个字符串s和p,判断s是否能用p进行匹配. [题解] dp[i][j]表示s的前i个是否能被p的前j个匹配. 首先可以分成3大类情况,我们先从简单的看起: (1)s[i - 1] = p[j - 1],易得dp[i][j] = dp[i-1][j-1] (2)p[i - 1] = '.',因为'.'可以匹配任何字符,所以dp[i][j] = dp[i-1][j-1] (3)p[i - 1] = '*',这种情况就比较复杂了. 当p[j - 2] != s[i - 1] &&…

题目: Given a linked list, return the node where the cycle begins. If there is no cycle, return null. 思路: 第一次相遇时slow走过的距离:a+b,fast走过的距离:a+b+c+b. 因为fast的速度是slow的两倍,所以fast走的距离是slow的两倍,有 2(a+b) = a+b+c+b,可以得到a=c(这个结论很重要!). 我们发现L=b+c=a+b,也就是说,从一开始到二者第一次相遇,…

这个题真是坑死了,只怪自己不好吧.一开始审题,以为是给定一个首尾相连的链表,查看其中是否有循环(原谅我的无知吧!!).然后在那写啊写啊写,还在纠结是局部循环还是前一半和后一半一样这样的循环,blah blah....,此处省略各种YY.最后发现其实就是给定一个链表的头,判断这个链表是否环! 1.用while循环,如果碰到next指向null,则返回False.这种做法说得通,但是前提是你也得有null才行啊,如果是个环,就是个无限循环啊! 2.一个指针不行,那就用2个指针,主要就是解决上面无限循…

这道题是LeetCode里的第141道题. 题目要求: 给定一个链表,判断链表中是否有环. 进阶: 你能否不使用额外空间解决此题? 简单题,但是还是得学一下这道题的做法,这道题是用双指针一个fast,一个slow.fast每一步前进两个节点,slow前进一个节点.判断fast和slow是否相等或者为空就行了. 贴个代码: /** * Definition for singly-linked list. * struct ListNode { * int val; * ListNode *next…

Given an index k, return the kth row of the Pascal's triangle. For example, given k = 3, Return [1,3,3,1]. Note: Could you optimize your algorithm to use only O(k) extra space? 注意:只开辟O(K)的内存空间,所以只保存前一组数列和当前数组,并不断滚动更新即可. class Solution { public: vecto…

作者: 负雪明烛 id: fuxuemingzhu 个人博客: http://fuxuemingzhu.cn/ 目录 题目描述 题目大意 解题方法 回溯法 日期 题目地址:https://leetcode.com/problems/n-queens/ 题目描述 The n-queens puzzle is the problem of placing n queens on an n×n chessboard such that no two queens attack each other.…

作者: 负雪明烛 id: fuxuemingzhu 个人博客: http://fuxuemingzhu.cn/ 题目地址: https://leetcode.com/problems/h-index/description/ 题目描述: Given an array of citations (each citation is a non-negative integer) of a researcher, write a function to compute the researcher's…

Given a linked list, reverse the nodes of a linked list k at a time and return its modified list. If the number of nodes is not a multiple of k then left-out nodes in the end should remain as it is. You may not alter the values in the nodes, only nod…

Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists. 思路:使用伪头部 class Solution { public: ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) { ListNode fakehead();…

原题 Given two strings s and t which consist of only lowercase letters. String t is generated by random shuffling string s and then add one more letter at a random position. Find the letter that was added in t. Example: Input: s = "abcd" t = "…

Contains Duplicate Given an array of integers, find if the array contains any duplicates. Your function should return true if any value appears at least twice in the array, and it should return false if every element is distinct. 思路:简单题用set bool cont…

BUPT2017 wintertraining(16) #4 G HDU - 5831 题意 给定括号序列,问能否交换一对括号使得括号合法. 题解 注意()是No的情况. 任意时刻)不能比(超过2个以上. 最后)和(的差距要在两个以内,且n必须是偶数. 代码 #include <cstdio> #include <cstring> #include <algorithm> #include <iostream> using namespace std; ch…

Given a string s, reverse the string according to the following rules: All the characters that are not English letters remain in the same position. All the English letters (lowercase or uppercase) should be reversed. class Solution { public: string r…

[LeetCode]731. My Calendar II 解题报告(Python) 作者: 负雪明烛 id: fuxuemingzhu 个人博客: http://fuxuemingzhu.cn/ 题目地址:https://leetcode.com/problems/my-calendar-ii/description/ 题目描述: Implement a MyCalendarTwo class to store your events. A new event can be added if…

[LeetCode]137. Single Number II 解题报告(Python) 标签: LeetCode 题目地址:https://leetcode.com/problems/single-number-ii/description/ 题目描述: Given an array of integers, every element appears three times except for one, which appears exactly once. Find that singl…

[LeetCode]227. Basic Calculator II 解题报告(Python) 标签(空格分隔): LeetCode 作者: 负雪明烛 id: fuxuemingzhu 个人博客: http://fuxuemingzhu.me/ 题目地址:https://leetcode.com/problems/basic-calculator-ii/description/ 题目描述: Implement a basic calculator to evaluate a simple exp…