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DEQUE(Double-Ended QUEue)

Linear list in which elements can be added or removed at either endbut not in the middle

DEQUE can be implemented using

− Circular array

− Doubly linked list

here are two variations o! a de"ue

−

#nput restricted de"ue$ insertions at one end o! the list but allowsdeletion at both ends o! the list

− %utput restricted de"ue$ allows deletion at only one end o! the listbut insertions at both ends o! the list

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Using circular array

 &ssume DEQUE'*#+E, is a circular array with two pointers LEand .#/01 which points to two ends o! the "ueue

he condition LE23ULL is used to indicate that a de"ue is empty

Operations

  E3QDQ.$ o insert an item at the right end o! the "ueue

  4 E3QDQL$ o insert an item at le!t end o! the "ueue

  5 DEQDQ.$ o remove item !rom the right "ueue

  6 DEQDQL$ o remove item !rom le!t o! the "ueue

Input restricted dequeue (operations 1or 2, 3, 4)

Output restricted dequeue (operations1,2,3 or 4)

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ENQDQR – Insertions to the right end

Input: ITEM to be inserted at the right, let and right are t!o pointers

Output: ITEM inserted in the queue i not ull

"ata #tructure: "E$%E%E being the circular arra& representation

#teps:

  #!((le!t2 &3D right2*#+E) %.(right72le!t)

  8rint 9de"ue is !ull3o insertion:

E;it

  Else

  #! (right2<) then

  Le!t2 .ight2

  Else .ight2(right)=*#+E7

  Endi! 

  &'right,2#E>

  Endi! 

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ENQDQL- Insertions to the left end

Input: ITEM to be inserted at let, let and right are t!o pointers

Output: ITEM inserted in the let side o the "E$%E%E

"ata #tructure: "E$%E%E being the circular arra& representation

#teps

  #! (le!t2 &3D right2si?e) %. (le!t-2right)

  8rint(Queue over!low3% insertion) and E;it

  Else

  #!(le!t2<) then Le!t2.ight2

  Else #! (le!t2) then Le!t2si?e

  Else Le!t2le!t-

  End#! 

  Endi! 

  &'le!t,2#E>

  Endi!

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DEQDQL-Deletions from left end

Input: ' "E$%E%E !ith eleents

Output: The ite is deleted ro the let end

"ata #tructure: "E$%E%E being the circular arra& representation

#teps:

  #! (le!t2< &3D right2<) then

  8rint(9De"ueue Under!low:)

  Else #!(le!t2right) then

  #E>2&'le!t,

  Le!t2right2<

  Else  #E>2&'le!t,

  *et+(let)si-e.1

  End#! 

  End#! 

DEQDQR D l ti f i ht d

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DEQDQR-Deletions from right end

Input: ' "E$%E%E !ith eleents

Output: The ite is deleted ro the right end

"ata #tructure: "E$%E%E being the circular arra& representation

#teps:

  #! (le!t2< &3D right2<) then 8rint(9De"ueue Under!low:)

 Else #!(le!t2right) then

  #E>2&'right,

  Le!t2right2<

  Else

  #E>2&'right,

  #! (right2<) .ight2si?e

  Else .ight2right-

  End#! 

  End#! 

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eg Consider a DEQUE having @ memory cells

LE241 .#/026 DEQUE$ AAAA1 &1C1D1 AAA1 AAA 

is added to the right o! de"ue

wo letters on the right are deleted

B1L and > are added to the le!t o! the de"ue %ne letter on the le!t is deleted

. is added to the le!t o! de"ue

* is added to the right o! de"ue

is added to the right o! de"ue