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>> MARK GROZEN-SMITH: Hi, Mwen Mak
Grozen-Smith, e sa se Quicksort.

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Jis tankou sòt ensèsyon ak ti wonn
sòt, Quicksort se yon algorithm pou

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Fouye yon lis oswa yon etalaj de bagay sa yo.

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Pou senplisite, se pou yo asime ke moun ki
bagay sa yo yo se jis nonb antye relatif, men

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konnen ke Quicksort ap travay pou
pi plis pase jis chif yo.

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QuickStart se yon ti jan pi konplike
pase ensèsyon oswa ti wonn, men li la

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tou pi plis efikas
nan pifò ka.

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Tann yon dezyèm fwa.

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Èske li jis di "ki pi
ka yo, "pa" tout "?

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Enteresan, pa gen okenn.

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Se pa tout ka yo se menm bagay la.

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pa enkyete sou detay sa a si ou
pa gen pou wè gwo notasyon O ankò, men

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Quicksort se yon O (n okib) algorithm
nan pi move, jis tankou

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ensèsyon oswa sòt ti wonn.

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Sepandan, li tipikman aji pi plis
tankou yon fin vye granmoun analòg m algorithm.

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Pou ki sa?

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Nou pral jwenn tounen nan ki pita.

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Men, pou kounye a, kite yo jis aprann
ki jan Quicksort travay.

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>> Se konsa, kite a mache nan Quicksorting sa a
etalaj de nonb antye relatif soti nan pi piti

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rive pi gran.

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Isit la nou gen nonm antye relatif yo, 6,
5, 1, 3, 8, 4, 7, 9, ak 2.

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Premyèman, nou chwazi eleman final la nan
etalaj sa a - nan ka sa a, de -

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epi rele ke "pivot la." Apre sa, nou
kòmanse fè yon gade nan de bagay sa yo -

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yon sèl, endèks la ki pi ba, ki mwen pral refere
yo kòm rete a dwat a

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miray ranpa a nan, ak, de, leftmost a
eleman, ki mwen pral rele "aktyèl la

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eleman. "Ki sa nou pral fè se
gade tout lòt eleman yo, lòt

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pase pivot la, li mete tout eleman ki
ki pi piti pase pivot a nan

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rete nan miray la ak tout moun sa yo
pi gwo pase pivot a nan

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dwat Bondye ki gen miray la.

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Lè sa a, finalman, nou pral lage pivot la
dwa sou epi miray ranpa lavil yo mete l 'ant

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tout nimewo yo ki pi piti pase sa li
ak tout nimewo yo ki pi gwo.

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>> Se konsa, kite a fè sa.

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Ranmase 2 a, mete miray lavil la nan la
kòmanse, epi rele 6 "aktyèl la nan

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eleman. "Se konsa, nou vle gade nan
eleman aktyèl nou an, 6 la.

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E depi li nan pi gwo pase a
2, nou kite l 'gen yo nan

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dwat Bondye ki gen miray la.

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Lè sa a, nou deplase sou gade nan 5 an kòm nou
kounye a eleman ak wè ke sa a

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se, ankò, pi gwo pase pivot a, se konsa nou
kite li kote li se sou bò dwat la

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bò nan miray la.

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Nou deplase sou.

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Eleman aktyèl nou se
kounye a 1, ak - o.

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Sa a se diferan kounye a.

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Eleman ki la kounye a se kounye a pi piti pase
pivot a, se konsa nou vle mete l '

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nan kite nan miray la.

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Pou fè sa, kite yo jis chanje a
kounye a eleman ak endèks la ki pi ba

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chita jis sou bò dwat la nan miray la.

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Koulye a, nou deplase miray ranpa a nan moute yon sèl endèks
Se konsa, 1 a se sou bò gòch la

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bò nan miray ranpa a nan kounye a.

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>> Rete tann.

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Mwen jis mele eleman yo sou la
dwa bò nan miray la, pa t 'mwen menm?

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pa enkyete.

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Sa a amann.

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Bagay la sèlman nou pran swen sou pou kounye a
se ke tout eleman sa yo nan

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dwat Bondye ki gen miray ranpa a nan yo se pi gwo
pase pivot la.

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Pa gen okenn lòd aktyèl sipoze ankò.

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>> Koulye a, tounen nan klasman.

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Se konsa, nou kontinye sou gade nan
rès la nan eleman yo.

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Apre sa, nan ka sa a, nou wè ke gen
pa gen okenn lòt eleman pi piti pase nan

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pivot, se konsa nou kite yo tout sou
bò dwat la nan miray la.

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Finalman, nou jwenn yo eleman aktyèl la
ak wè ke li se pivot la.

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Koulye a, sa vle di nou gen de
seksyon nan etalaj la, yo te nan premye

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ti sou pivot a ak sou bò gòch la
nan miray la, ak dezyèm ke yo te nan

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pi gwo pase pivot a nan
dwa bò nan miray la.

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Nou vle mete eleman ki Thorne ant
de a, ak Lè sa a, nou pral konnen

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ki Thorne an se nan dwa li
final tri plas li.

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Se konsa, nou chanje eleman nan premye sou la
dwa bò nan miray la ak Thorne an,

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e nou konnen pivot la
nan yon pozisyon dwat li yo.

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>> Nou Lè sa a, repete pwosesis sa a pou la
subarrays kite la ak dwa nan pivot la.

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Depi dènye subarray a se sèlman yon sèl
eleman lontan, nou konnen sa a, se deja

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tri paske ki jan ou ka gen soti nan
lòd si w ap sèlman yon sèl eleman?

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Pou bò dwat la nan subarray a, nou
wè ke pivot a se 5, ak miray ranpa a

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se jis rete nan 6 la.

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Apre sa, eleman aktyèl la tou
kòmanse deyò tankou 6 la.

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Se konsa, 6, pi konsekan pase 5.

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Nou kite li kote li se sou la
dwa bò nan miray la.

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Koulye a, k ap deplase sou li a, 3 se mwens pase 5.

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Se konsa, nou chanje li ak eleman nan premye
jis dwa nan miray la.

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Koulye a, mwen te deplase miray ranpa a nan moute yon sèl.

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Koulye a, deplase sou a 8 an.

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8 ki pi gran pase 5,
Se konsa nou kite li.

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4 a se mwens pase 5, se konsa nou chanje li.

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Yo, epi sou.

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Yo, epi sou.

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>> Chak fwa nou repete pwosedi an sou la
kote kite la ak dwa nan etalaj la. nou

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chwazi yon pivot epi fè konparezon yo
ak kreye yon lòt nivo nan kite ak

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subarrays dwat.

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Sa a rele repetitif yo ap kontinye jouk
nou rive nan fen an lè nou te gen

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divize moute etalaj la an jeneral nan
jis subarrays longè 1.

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Soti nan la, nou konnen se etalaj la klase
paske chak eleman gen, nan

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kèk pwen, te yon pivot.

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Nan lòt mo, pou chak eleman yo, tout
chif yo sou bò goch la se pi piti

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valè ak tout nimewo yo nan la
dwa gen plis valè.

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>> Metòd sa a ap travay trè byen si la
valè de pivot a chwazi a se

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apeprè nan mitan an
ran de lis valè yo.

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Sa a ta vle di ke, apre yo fin nou avanse
eleman alantou li, gen sou kòm anpil

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eleman nan kite nan pivot la
kòm gen sou bò dwat la.

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Ak nati a divize-ak-konkeri nan la
Se Quicksort algorithm Lè sa a, pran

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anpil avantaj de.

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Sa vin kreye yon ègzekutabl nan O (n ale n,)
n la paske nou dwe fè n mwens 1

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konparezon sou chak moun k'ap viv koulye ak boutèy
n paske nou gen divize lis la

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louvri sesyon fwa n.

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Sepandan, nan ka yo pi mal, sa a
algorithm ka aktyèlman ap O (n

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okib.) Sipoze sou chak jenerasyon,
pivot a jis pou k ap pase yo nan

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pi piti oswa pi gwo a nan la
nimewo nou ap klasman.

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Sa ta vle di kraze desann lis la
n fwa, epi li fè n mwens 1

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konparezon chak fwa sèl.

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Se konsa, o nan n okib.

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>> Ki jan nou ka amelyore metòd la?

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Yon bon fason yo amelyore metòd la se
nan koupe desann sou pwobabilite ki genyen pou

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ègzekutabl a se tout tan tout tan aktyèlman
o nan n okib.

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Sonje sa a pi move, senaryo ka pi mal
ka sèlman rive lè a

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pivot chwazi se toujou pi wo a
oswa ki pi ba valè nan etalaj la.

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Pou asire sa a se mwens chans rive,
nou ka jwenn pivot a pa

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w ap chwazi eleman miltip ak
pran valè medyàn.

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>> Non mwen se Mak Grozen-Smith,
ak sa a se CS50.

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>> Pou senplisite, se pou yo asime ke moun ki
bagay sa yo yo se jis nonb antye relatif, men

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konnen ke Quicksert -

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Quicksert?

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M regrèt.

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>> Isit la nou gen nonm antye relatif yo
, 6, 5, 1, 3, 8, 4, 9.

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>> Oratè 1: Vrèman?

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>> Oratè 2: pa sispann la.

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>> Oratè 1: Vrèman?

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