teffi的专栏

Given a linked list, determine if it has a cycle in it.Follow up: Can you solve it without using extra space?解题思路：最好的方法是时间复杂度 O(n),空间复杂度 O(1) 的。设置两个指针,一个快一个慢,快 的指针每次走两步,慢的指针每次走一步,如果快指针和慢指针相遇,则说明有环。/**

描述 Given an integer, convert it to a roman numeral.Input is guaranteed to be within the range from 1 to 3999.分析 整数转换为罗马数字。 此题因为题目中明确了范围1到3999，所以用每位对应一个数组的方式较为简单。附罗马数字的规则：罗马数字共有7个，即I（1）、V（5）、X（10）、L（

描述 Given two binary strings, return their sum (also a binary string).For example, a = “11” b = “1” Return “100”.分析 翻转a和b，逐位相加求值，结果存入string中。 注意最高位的情况。代码class Solution {public: string addBina

描述 Given a string s consists of upper/lower-case alphabets and empty space characters ’ ‘, return the length of last word in the string.If the last word does not exist, return 0.Note: A word is define

描述 Write a function that takes a string as input and returns the string reversed.Example: Given s = “hello”, return “olleh”.分析 翻转字符串。 取字符串长度的一半进行循环，首尾交换。代码class Solution {public: string revers

Given a linked list, reverse the nodes of a linked list k at a time and return its modified list.k is a positive integer and is less than or equal to the length of the linked list. If the number of nod

Reverse a linked list from position m to n. Do it in-place and in one-pass.For example: Given 1->2->3->4->5->NULL, m = 2 and n = 4,return 1->4->3->2->5->NULL.Note: Given m, n satisfy the following co

Given a singly linked list where elements are sorted in ascending order, convert it to a height balanced BST./** * Definition for singly-linked list. * struct ListNode { * int val; * ListNo

Given a linked list, return the node where the cycle begins. If there is no cycle, return null.Note: Do not modify the linked list.Follow up: Can you solve it without using extra space?解题思路：当 fast 与 s

Given a singly linked list L: L0→L1→…→Ln-1→Ln, reorder it to: L0→Ln→L1→Ln-1→L2→Ln-2→…You must do this in-place without altering the nodes’ values.For example, Given {1,2,3,4}, reorder it to {1,4,2,3}

Given a singly linked list, determine if it is a palindrome.Follow up: Could you do it in O(n) time and O(1) space?分析： 反转链表法，将链表后半段原地翻转，再将前半段、后半段依次比较，判断是否相等，时间复杂度O（n），空间复杂度为O（1）满足题目要求。/** * Definit

描述 Given a roman numeral, convert it to an integer.Input is guaranteed to be within the range from 1 to 3999.分析 罗马数字转换为整数。附罗马数字的规则：罗马数字共有7个，即I（1）、V（5）、X（10）、L（50）、C（100）、D（500）和M（1000）。 1、重复数次：一个罗马数

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 p

Given a linked list and a value x, partition it such that all nodes less than x come before nodes greater than or equal to x.You should preserve the original relative order of the nodes in each of the

Write a function to delete a node (except the tail) in a singly linked list, given only access to that node.Supposed the linked list is 1 -> 2 -> 3 -> 4 and you are given the third node with value 3, t

Given a singly linked list, group all odd nodes together followed by the even nodes. Please note here we are talking about the node number and not the value in the nodes.You should try to do it in plac

Remove all elements from a linked list of integers that have value val.Example Given: 1 –> 2 –> 6 –> 3 –> 4 –> 5 –> 6, val = 6 Return: 1 –> 2 –> 3 –> 4 –> 5Credits: Special thanks to @mithmatt for a

Given a sorted linked list, delete all nodes that have duplicate numbers, leaving only distinct numbers from the original list.For example, Given 1->2->3->3->4->4->5, return 1->2->5. Given 1->1->1->2

Merge k sorted linked lists and return it as one sorted list. Analyze and describe its complexity.解题思路：复用21. Merge Two Sorted Lists的代码，链表两两合并，直到最后只剩一个链表为止。/** * Definition for singly-linked list. * s

描述 Implement next permutation, which rearranges numbers into the lexicographically next greater permutation of numbers.If such arrangement is not possible, it must rearrange it as the lowest possible

描述 Given a collection of distinct numbers, return all possible permutations.For example, [1,2,3] have the following permutations:[ [1,2,3], [1,3,2], [2,1,3], [2,3,1], [3,1,2], [3,2,1]]分析

描述 The set [1,2,3,…,n] contains a total of n! unique permutations.By listing and labeling all of the permutations in order, We get the following sequence (ie, for n = 3):“123” “132” “213” “231” “

描述 You are given an n x n 2D matrix representing an image.Rotate the image by 90 degrees (clockwise).Follow up: Could you do this in-place?分析 首先想到，纯模拟，从外到内一圈一圈的转，但这个方法太慢。 如下图，首先沿着副对角线翻转一次，然后沿着水平中

Given a non-negative integer represented as a non-empty array of digits, plus one to the integer.You may assume the integer do not contain any leading zero, except the number 0 itself.The digits are st

描述 Given an array S of n integers, find three integers in S such that the sum is closest to a given number, target. Return the sum of the three integers. You may assume that each input would have exac

Write a program to find the node at which the intersection of two singly linked lists begins.For example, the following two linked lists:A: a1 → a2 ↘ c1

Reverse a singly linked list.解题思路：利用头节点，头插法。/** * Definition for singly-linked list. * struct ListNode { * int val; * ListNode *next; * ListNode(int x) : val(x), next(NULL) {} * }; *

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.解题思路：合并两个有序列表。/** * Definition for singly-linked list. * s

Reverse digits of an integer.Example1: x = 123, return 321 Example2: x = -123, return -321分析：注意溢出情况。class Solution {public: int reverse(int x) { int digit = 0; int sign = 1;

Implement atoi to convert a string to an integer.Hint: Carefully consider all possible input cases. If you want a challenge, please do not see below and ask yourself what are the possible input cases.N

Determine whether an integer is a palindrome. Do this without extra space.分析： 1. 如果将数字转化为字符串，再判断是否为回文字符串，要使用额外空间，不满足要求。 2. 考虑将数字翻转，(LeetCode 7. Reverse Integer )[http://blog.youkuaiyun.com/teffi/article/d

Given a binary array, find the maximum number of consecutive 1s in this array.Example 1:Input: [1,1,0,1,1,1]Output: 3Explanation: The first two digits or the last three digits are consecutive 1s.

The string “PAYPALISHIRING” is written in a zigzag pattern on a given number of rows like this: (you may want to display this pattern in a fixed font for better legibility) P A H NA P L S I I G

题目： Given an array of integers, return indices of the two numbers such that they add up to a specific target.You may assume that each input would have exactly one solution.Example:Given nums = [2, 7,

描述Follow up for "Search in Rotated Sorted Array":What if duplicates are allowed?Would this affect the run-time complexity? How and why?Suppose an array sorted in ascending order is rotated at some piv

描述 Suppose an array sorted in ascending order is rotated at some pivot unknown to you beforehand.(i.e., 0 1 2 4 5 6 7 might become 4 5 6 7 0 1 2).You are given a target value to search. If found in th

Given an array of integers, every element appears twice except for one. Find that single one.Note: Your algorithm should have a linear runtime complexity. Could you implement it without using extra me

A linked list is given such that each node contains an additional random pointer which could point to any node in the list or null.Return a deep copy of the list.分析：见《剑指offer》面试题：26 复杂链表的复制。/** * Defi

Sort a linked list in O(n log n) time using constant space complexity.分析：归并排序，考虑链表和数组的区别，链表每次划分为两段链表，然后有序链表归并。可以复用21. Merge Two Sorted Lists代码。/** * Definition for singly-linked list. * struct ListNo

Sort a linked list using insertion sort./** * Definition for singly-linked list. * struct ListNode { * int val; * ListNode *next; * ListNode(int x) : val(x), next(NULL) {} * }; */cl