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程序设计与算法基础3优秀文档.ppt

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程序设计与算法基础(6)OutlineSearchingproblemagainOperationsonhashtablesIdealHashing[0]WhatCanGoWrong?WhatCanGoWrong?HashTableIssuesHashfunctionsHashFunctionsStringToNon-negativeIntegerStringToNonnegativeIntegerMapintoahomebucketUniformHashFunctionN=>thiskeyisnotintheDItry(word,n,m);Achangeofanybitintheinputwillcausethechangeofanybitintheoutputwith0.PageRankwasdevelopedatStanfordUniversitybyLarryPageandSergeyBrinforasearchengine->GoogleInformationneededUraniaU=4h=1{012345678}NeveroverflowifthecapacityofthelinearlistisunlimitedTheprobabilityoffalsepositivedecreasesasmincreasesCombinelinearprobingandchainingktoosmall=>probabilityofdifferentkeyshavingsamesignatureishigh.OpenaddressingputwordatthebeginningofwordListanddetachitshashvalue;TerpsichoreT=0h=2{2578}BuildinvertedindexesUraniaU=4h=1{012345678}UraniaU=2h=8*{02345678},notreserved)HashingByDivisionSelectingTheDivisorSelectingTheDivisorSelectingTheDivisorHashingbyfoldingAboutfoldingHashingbyaMid-squarefunctionHashingbyextractionHashingbyradixtransformationHashingbycryptographichashalgorithmsCollisionresolutionCollisionresolutionbyopeningaddressingOpeningaddressingLinearprobingLinearProbing–GetAndInsertQuadraticprobingRandomprobingDoublehashingPerformanceofopeningaddressingPerformanceOfLinearProbingExpectedPerformanceLinearProbing–DeleteLinearProbing–Delete(34)LinearProbing–Delete(29)HashTableDesignHashTableDesignCollisionresolution:chainingPutinpairswhosekeysare6,12,34,29,28,11,23,7,0,33,30,45Homebucket=key%17.ExpectedPerformanceCoalescedhashing(coalescedchaining)BucketaddressingPerfecthashfunctionsCichelli’smethodtoconstructaminimalperfecthashfunctionreserveh(word);Searchclusterforpair(ifany)tofillvacatedbucketThenewitemcanalsobestoredinanoverflowarea.TheselistscanbeusedtosolveBooleanqueries:Thekeyisdividedintoseveralparts,andthesepartsarecombinedorfoldedtogether(key,element),herekeymaybeastring,number,record,etc.Pickm(equaltodivisor)tobeaprimenumberoranoddnumberwithnoprimedivisorssmallerthan20Mapping(dependencygraph):betwe