Leetcode题解

LeetCode题解：Sqrt(x)

Sqrt(x)Implementintsqrt(intx).Computeandreturnthesquarerootofx.思路：可以考虑二分搜索，但是二分搜索的起点不好判定，不过可以预先计算好numeric_limits::max()的平方根，然后搜索，这样范围就缩小很多而且避免了溢出。有的人用long来计算，这样在很多情况下也是可以避免溢出的。但是必须注意的是C++标准中long的最大值可以
MagiSu·2013-11-29 05:00

LeetCode题解：Divide Two Integers

DivideTwoIntegersDividetwointegerswithoutusingmultiplication,divisionandmodoperator.思路：核心思想是：令试除数等于除数，如果试除数小于被除数，那么试除数乘二，商乘二，继续比较；否则试除数除二，商除二，被除数减去试除数，重新开始。这其实就是类似二分搜索法。题目有一些边边角角的特殊情况，比如int的最大值比最小值的绝对
MagiSu·2013-11-29 04:00

LeetCode题解：Evaluate Reverse Polish Notation

EvaluateReversePolishNotationEvaluatethevalueofanarithmeticexpressioninReversePolishNotation.Validoperatorsare+,-,*,/.Eachoperandmaybeanintegeroranotherexpression.Someexamples:["2","1","+","3","*"]->(
MagiSu·2013-11-28 03:00

LeetCode题解：Search in Rotated Sorted Array

SearchinRotatedSortedArraySupposeasortedarrayisrotatedatsomepivotunknowntoyoubeforehand.(i.e.,0124567mightbecome4567012).Youaregivenatargetvaluetosearch.Iffoundinthearrayreturnitsindex,otherwisereturn
MagiSu·2013-11-26 07:00

LeetCode题解： Search for a Range

SearchforaRangeGivenasortedarrayofintegers,findthestartingandendingpositionofagiventargetvalue.Youralgorithm'sruntimecomplexitymustbeintheorderofO(logn).Ifthetargetisnotfoundinthearray,return[-1,-1].F
MagiSu·2013-11-26 06:00

LeetCode题解：Surrounded Regions

SurroundedRegionsTotalAccepted:1947TotalSubmissions:13017Givena2Dboardcontaining'X'and'O',captureallregionssurroundedby'X'.Aregioniscapturedbyflippingall'O'sinto'X'sinthatsurroundedregion.Forexample,X
MagiSu·2013-11-26 04:00

LeetCode题解： Reverse Linked List II

ReverseLinkedListIIReversealinkedlistfrompositionmton.Doitin-placeandinone-pass.Forexample:Given1->2->3->4->5->NULL,m=2andn=4,return1->4->3->2->5->NULL.Note:Givenm,nsatisfythefollowingcondition:1≤m≤n≤
MagiSu·2013-11-26 03:00

LeetCode题解： Construct Binary Tree from Preorder and Inorder Traversal

ConstructBinaryTreefromPreorderandInorderTraversalGivenpreorderandinordertraversalofatree,constructthebinarytree.Note:Youmayassumethatduplicatesdonotexistinthetree.思路：和inorder+postorder的思路类似。题解：/** *D
MagiSu·2013-11-25 15:00

LeetCode题解： Construct Binary Tree from Inorder and Postorder Traversal

ConstructBinaryTreefromInorderandPostorderTraversalGiveninorderandpostordertraversalofatree,constructthebinarytree.Note:Youmayassumethatduplicatesdonotexistinthetree.思路：已知中序遍历和后序遍历的结果，要求反推树结构。首先注意到后序遍
MagiSu·2013-11-25 14:00

LeetCode题解：Decode Ways

DecodeWaysAmessagecontaininglettersfromA-Zisbeingencodedtonumbersusingthefollowingmapping:'A'->1 'B'->2 ... 'Z'->26 Givenanencodedmessagecontainingdigits,determinethetotalnumberofwaystodecodeit.Forexa
MagiSu·2013-11-25 13:00

LeetCode题解：Combinations

CombinationsGiventwointegersnandk,returnallpossiblecombinationsofknumbersoutof1...n.Forexample,Ifn=4andk=2,asolutionis:[ [2,4], [3,4], [2,3], [1,2], [1,3], [1,4], ] 思路：递归填数即可。题解：classSolution{ public:
MagiSu·2013-11-25 03:00

LeetCode题解：3sum closest

3SumClosestGivenanarraySofnintegers,findthreeintegersinSsuchthatthesumisclosesttoagivennumber,target.Returnthesumofthethreeintegers.Youmayassumethateachinputwouldhaveexactlyonesolution.Forexample,give
MagiSu·2013-11-25 00:00

LeetCode题解：Combination Sum I and II

CombinationSumGivenasetofcandidatenumbers(C)andatargetnumber(T),findalluniquecombinationsinCwherethecandidatenumberssumstoT.ThesamerepeatednumbermaybechosenfromCunlimitednumberoftimes.Note:Allnumbers(
MagiSu·2013-11-25 00:00

LeetCode题解：3Sum

3SumGivenanarraySofnintegers,arethereelementsa,b,cinSsuchthata+b+c=0?Findalluniquetripletsinthearraywhichgivesthesumofzero.Note:Elementsinatriplet(a,b,c)mustbeinnon-descendingorder.(ie,a≤b≤c)Thesoluti
MagiSu·2013-11-24 15:00

LeetCode题解：Merge/Insert Interval

MergeIntervalsGivenacollectionofintervals,mergealloverlappingintervals.Forexample,Given[1,3],[2,6],[8,10],[15,18],return[1,6],[8,10],[15,18].InsertIntervalGivenasetofnon-overlappingintervals,insertane
MagiSu·2013-11-24 06:00

LeetCode题解： Reverse Nodes in k-Group

ReverseNodesink-GroupGivenalinkedlist,reversethenodesofalinkedlistkatatimeandreturnitsmodifiedlist.Ifthenumberofnodesisnotamultipleofkthenleft-outnodesintheendshouldremainasitis.Youmaynotalterthevalue
MagiSu·2013-11-22 15:00

LeetCode题解： Regular Expression Matching

RegularExpressionMatchingImplementregularexpressionmatchingwithsupportfor'.'and'*'.'.'Matchesanysinglecharacter. '*'Matcheszeroormoreoftheprecedingelement. Thematchingshouldcovertheentireinputstring(
MagiSu·2013-11-22 08:00

LeetCode题解：String to Integer (atoi)

StringtoInteger(atoi)Implementatoitoconvertastringtoaninteger.Hint:Carefullyconsiderallpossibleinputcases.Ifyouwantachallenge,pleasedonotseebelowandaskyourselfwhatarethepossibleinputcases.Notes:Itisin
MagiSu·2013-11-19 05:00

LeetCode题解：ZigZag Conversion

ZigZagConversionThestring"PAYPALISHIRING"iswritteninazigzagpatternonagivennumberofrowslikethis:(youmaywanttodisplaythispatterninafixedfontforbetterlegibility)PAHN APLSIIG YIR Andthenreadlinebyline:"PA
MagiSu·2013-11-19 05:00

LeetCode题解：Longest Palindromic Substring

LongestPalindromicSubstringGivenastringS,findthelongestpalindromicsubstringinS.YoumayassumethatthemaximumlengthofSis1000,andthereexistsoneuniquelongestpalindromicsubstring.思路：只想到一个O(n^2)的解法。就是从左到右扫描起点
MagiSu·2013-11-19 04:00

LeetCode题解：Sort List

SortListSortalinkedlistinO(nlogn)timeusingconstantspacecomplexity.思路：其实可以做一个快速排序或者merge排序之类的。但是这里的linkedlist是单向的，所以普通的快排办不到。不过仍然可以借用快速排序的思路，每次把链表分为三个部分：小于pivot，等于pivot和大于pivot，然后对小于大于pivot的部分进行排序。最后把三
MagiSu·2013-11-19 02:00

LeetCode题解： Longest Consecutive Sequence

LongestConsecutiveSequenceGivenanunsortedarrayofintegers,findthelengthofthelongestconsecutiveelementssequence.Forexample,Given[100,4,200,1,3,2],Thelongestconsecutiveelementssequenceis[1,2,3,4].Returni
MagiSu·2013-11-17 09:00

LeetCode题解： Flatten Binary Tree to Linked List

FlattenBinaryTreetoLinkedListGivenabinarytree,flattenittoalinkedlistin-place.Forexample,Given1 /\ 25 /\\ 346 Theflattenedtreeshouldlooklike:1 \ 2 \ 3 \ 4 \ 5 \ 6 思路：按preorder顺序访问树，使用迭代的方式进行。这样迭代过程中下一个
MagiSu·2013-11-17 07:00

LeetCode题解： Binary Tree Zigzag Level Order Traversal

BinaryTreeZigzagLevelOrderTraversalGivenabinarytree,returnthezigzaglevelordertraversalofitsnodes'values.(ie,fromlefttoright,thenrighttoleftforthenextlevelandalternatebetween).Forexample:Givenbinarytre
MagiSu·2013-11-17 06:00

LeetCode题解：Longest Valid Parentheses

LongestValidParenthesesGivenastringcontainingjustthecharacters'('and')',findthelengthofthelongestvalid(well-formed)parenthesessubstring.For"(()",thelongestvalidparenthesessubstringis"()",whichhaslengt
MagiSu·2013-11-17 00:00

LeetCode题解：Set Matrix Zeroes

SetMatrixZeroesGivenamxnmatrix,ifanelementis0,setitsentirerowandcolumnto0.Doitinplace.思路：如果允许额外存储行和列清零的信息，那么题目就没有意思了。希望只用O(1)的存储空间的时候，我们就得利用矩阵本身的存储空间了：把清零的信息保存在第一行和第一列里。题解：classSolution{ public: voids
MagiSu·2013-11-16 12:00

LeetCode题解：Word Break I and II

WordBreakGivenastringsandadictionaryofwordsdict,determineifscanbesegmentedintoaspace-separatedsequenceofoneormoredictionarywords.Forexample,givens="leetcode",dict=["leet","code"].Returntruebecause"lee
MagiSu·2013-11-15 05:00

LeetCode题解：Rotate List

Rotate List Given a list, rotate the list to the right by k places, where k is non-negative. For example: Given 1->2->3->4->5->NULL and k = 2, return 4->5->1->2->3->NU
·2013-11-14 20:00

LeetCode题解：Rotate List

RotateListGivenalist,rotatethelisttotherightbykplaces,wherekisnon-negative.Forexample:Given1->2->3->4->5->NULLandk=2,return4->5->1->2->3->NULL.思路：这个题目的陷阱中，输入为空指针大约不算其中之一。主要的问题是k可以大于这个链表的总长度。这样，真正的旋转次数
MagiSu·2013-11-14 05:00

LeetCode题解：Insertion Sort List

InsertionSortListSortalinkedlistusinginsertionsort.思路：标准的插入排序。就是考察一下链表的操作而已。题解：/** *Definitionforsingly-linkedlist. *structListNode{ *intval; *ListNode*next; *ListNode(intx):val(x),next(NULL){} *}; */
MagiSu·2013-11-14 01:00

LeetCode题解：Add Two Numbers

AddTwoNumbersYouaregiventwolinkedlistsrepresentingtwonon-negativenumbers.Thedigitsarestoredinreverseorderandeachoftheirnodescontainasingledigit.Addthetwonumbersandreturnitasalinkedlist.Input:(2->4->3)
MagiSu·2013-11-13 13:00

LeetCode题解：Add Binary

AddBinaryGiventwobinarystrings,returntheirsum(alsoabinarystring).Forexample,a="11"b="1"Return"100".思路：小学编程竞赛题。题解：classSolution{ public: stringaddBinary(stringa,stringb){ boolcarry=false; constintLenA
MagiSu·2013-11-13 13:00

LeetCode题解：Longest Substring Without Repeating Characters

LongestSubstringWithoutRepeatingCharactersGivenastring,findthelengthofthelongestsubstringwithoutrepeatingcharacters.Forexample,thelongestsubstringwithoutrepeatinglettersfor"abcabcbb"is"abc",whichthele
MagiSu·2013-11-13 13:00

LeetCode题解：Longest Common Prefix

LongestCommonPrefixWriteafunctiontofindthelongestcommonprefixstringamongstanarrayofstrings.思路：两个字符串的LCP可以通过依次比较相应字符的方法来完成。题解：classSolution{ public: stringlcp(conststring&s1,conststring&s2) { constintL
MagiSu·2013-11-13 12:00

LeetCode题解： Validate Binary Search Tree

ValidateBinarySearchTreeGivenabinarytree,determineifitisavalidbinarysearchtree(BST).AssumeaBSTisdefinedasfollows:Theleftsubtreeofanodecontainsonlynodeswithkeyslessthanthenode'skey.Therightsubtreeofano
MagiSu·2013-11-13 12:00

LeetCode题解：Sort Colors

SortColorsGivenanarraywithnobjectscoloredred,whiteorblue,sortthemsothatobjectsofthesamecolorareadjacent,withthecolorsintheorderred,whiteandblue.Here,wewillusetheintegers0,1,and2torepresentthecolorred,
MagiSu·2013-11-13 12:00

LeetCode题解：N-Queens I and II

N-QueensThen-queenspuzzleistheproblemofplacingnqueensonann×nchessboardsuchthatnotwoqueensattackeachother.Givenanintegern,returnalldistinctsolutionstothen-queenspuzzle.Eachsolutioncontainsadistinctboar
MagiSu·2013-11-13 11:00

LeetCode题解：Palindrome Number

PalindromeNumberDeterminewhetheranintegerisapalindrome.Dothiswithoutextraspace.思路：题目要求只能用O(1)的空间，所以不能考虑把它转化为字符串然后reverse比较的方法。在提示中也提到了，如果考虑reversenumber的方法，可能造成溢出。那么只能选择分离数字的方法，当然要用到大量除10和取余数的计算。真是一件悲
MagiSu·2013-11-13 11:00

LeetCode题解：Remove Nth Node from End of List

RemoveNthNodeFromEndofListGivenalinkedlist,removethenthnodefromtheendoflistandreturnitshead.Forexample,Givenlinkedlist:1->2->3->4->5,andn=2. Afterremovingthesecondnodefromtheend,thelinkedlistbecomes1
MagiSu·2013-11-10 10:00

LeetCode题解：Minimum Path Sum

MinimumPathSumGivenamxngridfilledwithnon-negativenumbers,findapathfromtoplefttobottomrightwhichminimizesthesumofallnumbersalongitspath.Note:Youcanonlymoveeitherdownorrightatanypointintime.思路：一个简单的动态规划
MagiSu·2013-11-10 06:00

LeetCode题解：Gray Code

GrayCodeThegraycodeisabinarynumeralsystemwheretwosuccessivevaluesdifferinonlyonebit.Givenanon-negativeintegernrepresentingthetotalnumberofbitsinthecode,printthesequenceofgraycode.Agraycodesequencemust
MagiSu·2013-11-10 05:00

LeetCode题解：Convert Sorted Array to Binary Search Tree

ConvertSortedArraytoBinarySearchTreeGivenanarraywhereelementsaresortedinascendingorder,convertittoaheightbalancedBST.思路：给定一个区间[left,right]，取其中值mid=(left+right)/2对应的元素作为二叉树的根。二叉树的左子树根的值为对[left,mid-1]继续
MagiSu·2013-11-10 05:00

【LeetCode题解】Remove Element

RemoveElement （Java代码）MySubmissionsGivenanarrayandavalue,removeallinstancesofthatvalueinplaceandreturnthenewlength.Theorderofelementscanbechanged.Itdoesn'tmatterwhatyouleavebeyondthenewlength.publiccl
cc1044588189·2013-11-09 22:00

【LeetCode题解】Search Insert Position

SearchInsertPosition （Java代码）Givenasortedarrayandatargetvalue,returntheindexifthetargetisfound.Ifnot,returntheindexwhereitwouldbeifitwereinsertedinorder.Youmayassumenoduplicatesinthearray.Herearefewex
cc1044588189·2013-11-09 21:00

【LeetCode题解】Single Number II

SingleNumberII （Java代码）Givenanarrayofintegers,everyelementappears three timesexceptforone.Findthatsingleone.Note:Youralgorithmshouldhavealinearruntimecomplexity.Couldyouimplementitwithoutusingextramem
cc1044588189·2013-11-08 17:00

LeetCode题解：Partition List

PartitionListGivenalinkedlistandavaluex,partitionitsuchthatallnodeslessthanxcomebeforenodesgreaterthanorequaltox.Youshouldpreservetheoriginalrelativeorderofthenodesineachofthetwopartitions.Forexample,
MagiSu·2013-11-07 04:00

【LeetCode题解】Reverse Integer

ReverseInteger（Java代码）Reversedigitsofaninteger.Example1: x=123,return321Example2: x=-123,return-321clicktoshowspoilers.Haveyouthoughtaboutthis?Herearesomegoodquestionstoaskbeforecoding.Bonuspointsfory
cc1044588189·2013-11-06 00:00

【LeetCode题解】Single Number

SingleNumber（Java代码）Givenanarrayofintegers,everyelementappears twice exceptforone.Findthatsingleone.Note:Youralgorithmshouldhavealinearruntimecomplexity.Couldyouimplementitwithoutusingextramemory?解题思路
cc1044588189·2013-11-06 00:00

LeetCode题解：Symmetric Tree

SymmetricTreeGivenabinarytree,checkwhetheritisamirrorofitself(ie,symmetricarounditscenter).Forexample,thisbinarytreeissymmetric:1 /\ 22 /\/\ 3443 Butthefollowingisnot:1 /\ 22 \\ 33 Note:Bonuspointsify
MagiSu·2013-11-05 16:00

LeetCode题解：Climbing Stairs

ClimbingStairsYouareclimbingastaircase.Ittakesnstepstoreachtothetop.Eachtimeyoucaneitherclimb1or2steps.Inhowmanydistinctwayscanyouclimbtothetop?思路：这种题目属于那种“想到解法就很快做出”的类型。n个台阶的楼梯的走法等于：ways(n)=ways(n-1)
MagiSu·2013-11-05 15:00
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