﻿
482
Chapter10 AlgorithmDesignTechniques
M
1
=(A
1,2
A
2,2
)(B
2,1
+B
2,2
)
M
2
=(A
1,1
+A
2,2
)(B
1,1
+B
2,2
)
M
3
=(A
1,1
A
2,1
)(B
1,1
+B
1,2
)
M
4
=(A
1,1
+A
1,2
)B
2,2
M
5
=A
1,1
(B
1,2
B
2,2
)
M
6
=A
2,2
(B
2,1
B
1,1
)
M
7
=(A
2,1
+A
2,2
)B
1,1
C
1,1
=M
1
+M
2
M
4
+M
6
C
1,2
=M
4
+M
5
C
2,1
=M
6
+M
7
C
2,2
=M
2
M
3
+M
5
M
7
Itisstraightforwardtoverifythatthistrickyorderingproducesthedesiredvalues.The
runningtimenowsatisﬁestherecurrence
T(N)=7T(N/2)+O(N
2
)
ThesolutionofthisrecurrenceisT(N)=O(Nlog
2
7)=O(N2.81).
Asusual,therearedetailstoconsider,suchasthecasewhenNisnotapowerof2,
butthesearebasicallyminornuisances.Strassen’salgorithmisworsethanthestraightfor-
wardalgorithmuntilNisfairlylarge.Itdoesnotgeneralizeforthecasewherethematrices
aresparse(containmanyzeroentries),anditdoesnoteasilyparallelize.Whenrunwith
ﬂoating-pointentries,itislessstablenumericallythantheclassicalgorithm.Thus,until
tanttheoreticalmilestoneandcertainlyshowsthatincomputerscience,asinmanyother
ﬁelds,eventhoughaproblemseemstohaveanintrinsiccomplexity,nothingiscertain
untilproven.
10.3 DynamicProgramming
Intheprevioussection,wesawthataproblemthatcanbemathematicallyexpressedrecur-
sivelycanalsobeexpressedasarecursivealgorithm,inmanycasesyieldingasigniﬁcant
performanceimprovementoveramorenaïveexhaustivesearch.
Anyrecursivemathematicalformulacouldbedirectlytranslatedtoarecursivealgo-
rithm, butthe underlying g reality is that often the compiler willnot dojustice tothe
recursivealgorithm,andaninefﬁcientprogramresults.Whenwesuspectthatthisislikely
tobethecase,wemustprovidealittlemorehelptothecompiler,byrewritingtherecursive
problemsinatable.Onetechniquethatmakesuseofthisapproachisknownasdynamic
programming.
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10.3 DynamicProgramming
483
InChapter2,wesawthatthenaturalrecursiveprogramtocomputetheFibonaccinumbers
isveryinefﬁcient.RecallthattheprogramshowninFigure10.40hasarunningtime,T(N),
thatsatisﬁesT(N)≥T(N−1)+T(N−2).SinceT(N)satisﬁesthesamerecurrencerelation
astheFibonaccinumbersandhasthesameinitialconditions,T(N)infactgrowsatthe
samerateastheFibonaccinumbersandisthusexponential.
Ontheotherhand,sincetocomputeF
N
,allthatisneededisF
N−1
andF
N−2
,weonly
needtorecordthetwomostrecentlycomputedFibonaccinumbers.ThisyieldstheO(N)
algorithminFigure10.41.
1
/**
2
* Compute Fibonacci i numbers as described d in Chapter 1.
3
*/
4
long long fib( ( int n n )
5
{
6
if( n n <= 1 1 )
7
return 1;
8
else
9
return fib( n - - 1 1 ) + fib( ( n n - 2 );
10
}
Figure10.40 InefﬁcientalgorithmtocomputeFibonaccinumbers
1
/**
2
* Compute Fibonacci i numbers as described d in Chapter 1.
3
*/
4
long long fibonacci( int t n )
5
{
6
if( n n <= 1 1 )
7
return 1;
8
9
long long last = = 1;
10
long long last nextToLast = = 1;
11
long long answer r = = 1;
12
13
for( int t i i = 2; i <= n; ++i i )
14
{
15
answer = last + + nextToLast;
16
nextToLast = = last;
17
18
}
19
20
}
Figure10.41 LinearalgorithmtocomputeFibonaccinumbers
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484
Chapter10 AlgorithmDesignTechniques
Thereasonthattherecursivealgorithmissoslowisbecauseofthealgorithmusedto
simulaterecursion.TocomputeF
N
,thereisonecalltoF
N−1
andF
N−2
.However,since
F
N−1
recursively makes acalltoF
N−2
and F
N−3
,thereareactually twoseparatecalls
tocomputeF
N−2
.Ifonetracesouttheentirealgorithm,thenwecanseethatF
N−3
is
computedthreetimes,F
N−4
iscomputedﬁvetimes,F
N−5
iscomputedeighttimes,and
soon.AsFigure10.42shows,thegrowthofredundantcalculationsisexplosive.Ifthe
compiler’srecursionsimulationalgorithmwereabletokeepalistofallprecomputedvalues
explosionwouldbeavoided.ThisiswhytheprograminFigure10.41issomuchmore
efﬁcient.
As asecond example,wesaw in Chapter7 how tosolvethe recurrence C(N) =
(2/N)
N−1
i=0
C(i)+N, with C(0) = = 1.Supposethat t we want tocheck, numerically,
whetherthesolutionweobtainediscorrect.Wecould thenwritethesimpleprogram
inFigure10.43toevaluatetherecursion.
Onceagain,therecursivecallsduplicatework.Inthiscase,therunningtimeT(N)
satisﬁesT(N)=
N−1
i=0
T(i)+N,because,asshowninFigure10.44,thereisone(direct)
seenthetreeshowninFigure10.44?).SolvingforT(N),weﬁndthatitgrowsexponentially.
F1
F0
F2
F1
F3
F1
F0
F2
F4
F1
F0
F2
F1
F3
F5
F1
F0
F2
F1
F3
F1
F0
F2
F4
F6
Figure10.42 TraceoftherecursivecalculationofFibonaccinumbers
1
double eval( ( int n )
2
{
3
if( n == 0 0 )
4
return 1.0;
5
else
6
{
7
double sum m = 0.0;
8
9
for( int i = 0; i i < < n; ++i )
10
sum += eval( i );
11
return 2.0 0 * sum / n n + + n;
12
}
13
}
Figure10.43 RecursivefunctiontoevaluateC(N)=2/N
N−1
i=0
C(i)+N
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10.3 DynamicProgramming
485
C1
C0
C2
C0
C3
C1
C0
C0
C4
C1
C0
C2
C0
C1
C0
C0
C5
C1
C0
C2
C0
C3
C1
C0
C0
C1
C0
C2
C0
C1
C0
C0
Figure10.44 Traceoftherecursivecalculationineval
1
double eval( ( int t n )
2
{
3
vector<double> c( n n + + 1 );
4
5
c[ 0 ] = = 1.0;
6
for( int t i i = 1; i <= n; ++i i )
7
{
8
double sum = = 0.0;
9
10
for( int t j = = 0; j j < i; ++j j )
11
sum += = c[ j j ];
12
c[ i i ] ] = = 2.0 0 * * sum m / i i + + i;
13
}
14
15
return c[ n n ];
16
}
Figure10.45 EvaluatingC(N)=2/N
N−1
i=0
C(i)+Nwithatable
Byusingatable,weobtaintheprograminFigure10.45.Thisprogramavoidstheredun-
dantrecursivecallsandrunsinO(N
2
).Itisnotaperfectprogram;asanexercise,you
shouldmakethesimplechangethatreducesitsrunningtimetoO(N).
10.3.2 OrderingMatrixMultiplications
Supposewearegivenfourmatrices,A,B,C,andD,ofdimensions= 50×10,=
10×40,C=40×30,andD=30×5.Althoughmatrixmultiplicationisnotcommutative,
itisassociative,whichmeansthatthematrixproductABCDcanbeparenthesized,and
thusevaluated,inanyorder.Theobviouswaytomultiply twomatricesofdimensions
p×qandq×r,respectively,usespqrscalarmultiplications.(Usingatheoreticallysuperior
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486
Chapter10 AlgorithmDesignTechniques
algorithmsuch as Strassen’salgorithmdoesnotsigniﬁcantlyaltertheproblemwewill
consider,sowewillassumethisperformancebound.)Whatisthebestwaytoperformthe
threematrixmultiplicationsrequiredtocomputeABCD?
Inthecaseoffourmatrices,itissimpletosolvetheproblembyexhaustivesearch,
sincethereareonlyﬁvewaystoorderthemultiplications.Weevaluateeachcasebelow:
(A((BC)D)): Evaluating BC requires 10 0 ×40×30 0 = = 12,000 0 multiplications.
tional 10×30 ×5 5 = 1,500 0 multiplications, , for a a total l of 13,500. Evaluating
5=2,500multiplications,foragrandtotalof16,000multiplications.
(A(B(CD))):EvaluatingCDrequires40×30×5=6,000multiplications.Evaluating
5=2,000multiplications,foratotalof8,000.Evaluating(A(B(CD)))requires8,000
foragrandtotalof10,500multiplications.
((AB)(CD)):EvaluatingCDrequires40×30×5=6,000multiplications.Evaluating
ABrequires50×10×40=20,000multiplications.Evaluating((AB)(CD))requires
40×5=10,000multiplicationsforagrandtotalof36,000multiplications.
(((AB)C)D): Evaluating AB requires 50 0 ×10×40 0 = = 20,000 0 multiplications.
tional50×40×30 = = 60,000multiplications,foratotalof f 80,000.Evaluating
5=7,500multiplications,foragrandtotalof87,500multiplications.
((A(BC))D): Evaluating BC requires 10 0 ×40×30 0 = = 12,000 0 multiplications.
tional50×10×30 = = 15,000multiplications,foratotalof f 27,000.Evaluating
5=7,500multiplications,foragrandtotalof34,500multiplications.
Thecalculationsshowthatthebestorderingusesroughlyone-ninththenumberof
multiplicationsastheworstordering.Thus,itmightbeworthwhiletoperformafewcal-
culationstodeterminetheoptimalordering.Unfortunately,noneoftheobviousgreedy
strategies seems to work.Moreover, the number of possible orderings grows quickly.
Supposewe deﬁneT(N)tobethisnumber.ThenT(1) ) = T(2) ) = 1,T(3) = = 2,and
T(4)=5,aswehaveseen.Ingeneral,
T(N)=
N−1
i=1
T(i)T(Ni)
To see e this, , suppose that the matrices are A
1
,A
2
,...,A
N
, and d the e last multiplica-
tionperformedis(A
1
A
2
···A
i
)(A
i+1
A
i+2
···A
N
).ThenthereareT(i)waystocompute
(A
1
A
2
···A
i
)andT(Ni)waystocompute(A
i+1
A
i+2
···A
N
).Thus,thereareT(i)T(Ni)
waystocompute(A
1
A
2
···A
i
)(A
i+1
A
i+2
···A
N
)foreachpossiblei.
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10.3 DynamicProgramming
487
Thesolutionofthisrecurrenceisthewell-knownCatalannumbers,whichgrowexpo-
nentially.Thus,forlargeN,anexhaustivesearchthroughallpossibleorderingsisuseless.
Nevertheless,thiscountingargumentprovidesabasisforasolutionthatissubstantially
betterthanexponential.Letc
i
bethenumberofcolumnsinmatrixA
i
for1≤iN.Then
A
i
hasc
i−1
rows,sinceotherwisethemultiplicationsarenotvalid.Wewilldeﬁnec
0
tobe
thenumberofrowsintheﬁrstmatrix,A
1
.
Suppose m
Left,Right
is the e number of multiplications required d to o multiply
A
Left
A
Left+1
···A
Right−1
A
Right
.Forconsistency,m
Left,Left
= 0.Supposethelastmultipli-
cation is s (A
Left
···A
i
)(A
i+1
···A
Right
),where Left ≤ ≤ Right. Then the number of
multiplicationsused ism
Left,i
+m
i+1,Right
+c
Left−1
c
i
c
Right
.Thesethreetermsrepresent
themultiplicationsrequiredtocompute(A
Left
···A
i
),(A
i+1
···A
Right
),andtheirproduct,
respectively.
If wedeﬁne M
Left,Right
tobethenumberofmultiplicationsrequired in an optimal
ordering,then,ifLeft<Right,
M
Left,Right
=
min
Lefti<Right
{M
Left,i
+M
i+1,Right
+c
Left−1
c
i
c
Right
}
This equation implies s that t if we e have e an optimal l multiplication arrangement t of
A
Left
···A
Right
,thesubproblemsA
Left
···A
i
andA
i+1
···A
Right
cannotbeperformedsub-
optimally.This shouldbeclear,sinceotherwisewecouldimprovetheentireresultby
replacingthesuboptimalcomputationbyanoptimalcomputation.
Theformulatranslatesdirectlytoarecursiveprogram,but,aswehaveseeninthe
lastsection,suchaprogramwouldbeblatantlyinefﬁcient.However,sincethereareonly
approximatelyN
2
/2valuesofM
Left,Right
thateverneedtobecomputed,itisclearthata
tablecanbeusedtostorethesevalues.FurtherexaminationshowsthatifRightLeft=k,
thentheonlyvaluesM
x,y
thatareneededinthecomputationofM
Left,Right
satisfyyx<k.
Thistellsustheorderinwhichweneedtocomputethetable.
1,N
,thenwecanusetheideasfromtheshortest-pathalgorithmsinChapter9.
WheneverwemakeachangetoM
Left,Right
,werecordthevalueofithatisresponsible.This
givesthesimpleprogramshowninFigure10.46.
Althoughtheemphasis ofthischapterisnotcoding,itisworthnoting thatmany
programmerstendtoshorten variablenames toasingleletter.
c
,
i
,and
k
areused as
single-lettervariablesbecausethisagreeswiththenameswehaveusedinthedescription
ofthealgorithm,whichisverymathematical.However,itisgenerallybesttoavoid
l
asa
variablename,because
l
lookstoomuchlike
1
andcanmakeforverydifﬁcultdebugging
ifyoumakeatranscriptionerror.
Returningtothealgorithmicissues,thisprogramcontainsatriplynestedloopandis
easilyseentoruninO(N
3
)time.Thereferencesdescribeafasteralgorithm,butsincethe
timetoperformtheactualmatrixmultiplicationisstilllikelytobemuchlargerthanthe
timetocomputetheoptimalordering,thisalgorithmisstillquitepractical.
10.3.3 OptimalBinarySearchTree
Ourseconddynamicprogrammingexampleconsidersthefollowinginput:Wearegiven
alistofwords,w
1
,w
2
,...,w
N
,andﬁxedprobabilities,p
1
,p
2
,...,p
N
,oftheiroccurrence.
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488
Chapter10 AlgorithmDesignTechniques
1
/**
2
* Compute optimal ordering of matrix multiplication.
3
* c contains the e number r of columns for each h of the e n matrices.
4
* c[ 0 ] is the number of rows in matrix x 1.
5
* The e minimum number r of multiplications is left in m[ 1 ][ n ].
6
* Actual ordering is computed d via a another procedure e using lastChange.
7
* m and lastChange are indexed starting at 1, instead of 0.
8
* Note: Entries below w main n diagonals of m and lastChange
9
* are e meaningless s and d uninitialized.
10
*/
11
void optMatrix( const vector<int> > & & c,
12
matrix<int> & m, matrix<int> > & & lastChange )
13
{
14
int n = = c.size( ( ) - 1;
15
16
for( int left t = = 1; ; left t <= n; ; ++left t )
17
m[ left ][ left ] ] = = 0;
18
for( int k k = = 1; k < n; ++k )
// k k is s right - left
19
for( int left = 1; left <= n n - - k; ; ++left t )
20
{
21
// For r each position
22
int right = left t + + k;
23
m[ left t ][ [ right ] ] = INFINITY;
24
for( int i i = = left; i < right; ; ++i i )
25
{
26
int thisCost t = = m[ [ left t ][ [ i i ] ] + m[ i + + 1 1 ][ right t ]
27
+ c[ left - - 1 ] * * c[ i i ] * c[ right ];
28
if( thisCost t < < m[ [ left t ][ [ right ] ] ) ) // / Update e min
29
{
30
m[ left t ][ right t ] ] = thisCost;
31
lastChange[ left t ][ [ right ] ] = = i;
32
}
33
}
34
}
35
}
Figure10.46 Programtoﬁndoptimalorderingofmatrixmultiplications
Theproblemistoarrangethesewordsinabinarysearchtreeinawaythatminimizes
theexpectedtotalaccesstime.Inabinarysearchtree,thenumberofcomparisonsneeded
toaccessanelementatdepthdisd+1,soifw
i
isplacedatdepthd
i
,thenwewantto
minimize
N
i=1
p
i
(1+d
i
).
Asanexample,Figure10.47showssevenwordsalongwiththeirprobabilityofoccur-
rence in n some e context. Figure 10.48 shows s three e possible binary search trees.Their
searchingcostsareshowninFigure10.49.
10.3 DynamicProgramming
489
Word
Probability
a
0.22
am
0.18
and
0.20
egg
0.05
if
0.25
the
0.02
two
0.08
Figure10.47 Sampleinputforoptimalbinarysearchtreeproblem
if
a
and
egg
two
the
am
if
a
and
egg
two
the
am
if
a
and
egg
two
the
am
Figure10.48 Threepossiblebinarysearchtreesfordatainprevioustable
Theﬁrsttreewasformedusingagreedystrategy.Thewordwiththehighestprobability
ofbeingaccessedwasplacedattheroot.Theleftandrightsubtreeswerethenformed
recursively.Thesecondtreeistheperfectlybalancedsearchtree.Neitherofthesetrees
Input
Tree#1
Tree#2
Tree#3
Word
Probability
AccessCost
AccessCost
AccessCost
w
i
p
i
Once
Sequence
Once
Sequence
Once
Sequence
a
0.22
2
0.44
3
0.66
2
0.44
am
0.18
4
0.72
2
0.36
3
0.54
and
0.20
3
0.60
3
0.60
1
0.20
egg
0.05
4
0.20
1
0.05
3
0.15
if
0.25
1
0.25
3
0.75
2
0.50
the
0.02
3
0.06
2
0.04
4
0.08
two
0.08
2
0.16
3
0.24
3
0.24
Totals
1.00
2.43
2.70
2.15
Figure10.49 Comparisonofthethreebinarysearchtrees
490
Chapter10 AlgorithmDesignTechniques
isoptimal,asdemonstratedbytheexistenceofthethirdtree.Fromthiswecanseethat
neitheroftheobvioussolutionsworks.
Thisisinitiallysurprising,sincetheproblemappearstobeverysimilartothecon-
greedyalgorithm.Constructionofanoptimalbinarysearchtreeisharder,becausethedata
arenotconstrainedtoappearonlyattheleaves,andalsobecausethetreemustsatisfythe
binarysearchtreeproperty.
wearetryingtoplacethe(sorted)wordsw
Left
,w
Left+1
,...,w
Right−1
,w
Right
intoabinary
searchtree.Supposetheoptimalbinarysearchtreehasw
i
astheroot,whereLeft≤ i
Right.Thentheleftsubtreemustcontainw
Left
,...,w
i−1
,andtherightsubtreemustcontain
w
i+1
,...,w
Right
(bythebinarysearchtreeproperty).Further,bothofthesesubtreesmust
alsobeoptimal,sinceotherwisetheycouldbereplacedbyoptimalsubtrees,whichwould
giveabettersolutionforw
Left
,...,w
Right
.Thus,wecanwriteaformulaforthecostC
Left,Right
IfLeft>Right,thenthecostofthetreeis0;thisisthe
nullptr
case,whichwealways
haveforbinarysearchtrees.Otherwise,therootcostsp
i
.Theleftsubtreehasacostof
C
Left,i−1
relativetoitsroot,andtherightsubtreehasacostofC
i+1,Right
relativetoitsroot.
AsFigure10.50shows,eachnodeinthesesubtreesisoneleveldeeperfromw
i
thanfrom
i−1
j=Left
p
j
and
Right
j=i+1
p
j
.Thisgivestheformula
C
Left,Right
=
min
LeftiRight
p
i
+C
Left,i−1
+C
i+1,Right
+
i−1
j=Left
p
j
+
Right
j=i+1
p
j
=
min
LeftiRight
C
Left,i−1
+C
i+1,Right
+
Right
j=Left
p
j
Fromthisequation,itisstraightforwardtowriteaprogramtocomputethecostofthe
optimalbinarysearchtree.Asusual,theactualsearchtreecanbemaintainedbysavingthe
valueofithatminimizesC
Left,Right
.Thestandardrecursiveroutinecanbeusedtoprintthe
actualtree.
w
i
w
Left
w
i−1
w
i+1
w
Right
Figure10.50 Structureofanoptimalbinarysearchtree
10.3 DynamicProgramming
491
Iteration=1
a..a
am..am
and..and
egg..egg
if..if
the..the
two..two
Left=1
Left=2
Left=3
Left=4
Left=5
Left=6
Left=7
.22
a
.18
am
.20
and
.05
egg
.25
if
.02
the
.08
two
Iteration=2
a..am
am..and
and..egg
egg..if
if..the
the..two
.58
a
.56
and
.30
and
.35
if
.29
if
.12
two
Iteration=3
a..and
am..egg
and..if
egg..the
if..two
1.02
am
.66
and
.80
if
.39
if
.47
if
Iteration=4
a..egg
am..if
and..the
egg..two
1.17
am
1.21
and
.84
if
.57
if
Iteration=5
a..if
am..the
and..two
1.83
and
1.27
and
1.02
if
Iteration=6
a..the
am..two
1.89
and
1.53
and
Iteration=7
a..two
2.15
and
Figure10.51 Computationoftheoptimalbinarysearchtreeforsampleinput
Figure10.51showsthetablethatwillbeproducedbythealgorithm.Foreachsub-
rangeofwords,thecostandrootoftheoptimalbinarysearchtreearemaintained.The
bottommostentrycomputestheoptimalbinarysearchtreefortheentiresetofwordsin
theinput.TheoptimaltreeisthethirdtreeshowninFigure10.48.
Theprecisecomputationfortheoptimalbinarysearchtreeforaparticularsubrange,
namely,am..if,isshowninFigure10.52.Itisobtainedbycomputingtheminimum-cost
treeobtainedbyplacingam,and,egg,andifattheroot.Forinstance,whenandisplacedat
theroot,theleftsubtreecontainsam..am(ofcost0.18,viapreviouscalculation),theright
subtreecontainsegg..if(ofcost0.35),andp
am
+p
and
+p
egg
+p
if
=0.68,foratotalcost
of1.21.
Therunning timeofthisalgorithmisO(N
3
),becausewhenitisimplemented,we
obtainatripleloop.AnO(N
2
)algorithmfortheproblemissketchedintheexercises.