Double Ended Queue Deque in DSA Go - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a Double Ended Queue (Deque)?

A Double Ended Queue, or Deque, is a data structure where elements can be added or removed from both the front and the back ends.

beginner

Name two main operations you can perform on both ends of a Deque.

You can perform insertion (add) and deletion (remove) operations at both the front and the back ends of a Deque.

beginner

How does a Deque differ from a regular Queue?

A regular Queue allows insertion at the back and removal from the front only, while a Deque allows insertion and removal at both ends.

beginner

What real-life example can help you understand a Deque?

Think of a line of people where you can join or leave from both the front and the back, like a double-ended waiting line.

intermediate

What is the time complexity of adding or removing elements from either end of a Deque?

Adding or removing elements from either end of a Deque is done in constant time, O(1), because it does not require shifting elements.

Which operation is NOT possible in a Deque?

A Add element at the front

B Remove element from the back

C Add element only at the back

D Remove element from the front

In a Deque, you can add elements at both the front and the back, not only at the back.

What is the main advantage of a Deque over a Queue?

A Supports insertion and removal at both ends

B Supports only insertion at the back

C Supports only removal from the front

D Has slower operations

A Deque allows insertion and removal at both ends, unlike a Queue which is limited to one end for insertion and the other for removal.

If you add elements 1, 2, 3 to the front of an empty Deque in that order, what will be the order when you remove from the front?

A 1, 2, 3

B 1, 3, 2

C 2, 3, 1

D 3, 2, 1

Adding 1, 2, 3 to the front means 3 is at the front, so removing from front gives 3, then 2, then 1.

What is the time complexity to remove an element from the back of a Deque?

A O(log n)

B O(1)

C O(n)

D O(n log n)

Removing an element from either end of a Deque is done in constant time O(1).

Which of these is a valid use case for a Deque?

A Undo functionality in text editors

B Sorting numbers

C Searching in a database

D Storing unique keys

Undo functionality often uses a Deque to add and remove actions from both ends efficiently.

Explain what a Double Ended Queue (Deque) is and how it works.

Think about a line where people can join or leave from both ends.

Describe the difference between a Queue and a Deque with examples.

Focus on where you can add and remove elements.

Practice

1. What is a Double Ended Queue (Deque)?

easy

A. A data structure where elements can be added or removed from both ends

B. A queue where elements can only be added at the rear

C. A stack where elements can only be added or removed from the top

D. A list that allows random access to elements

Solution

1. Step 1: Understand the definition of Deque

   A Deque allows insertion and deletion from both front and rear ends, unlike a queue or stack.

2. Step 2: Compare with other data structures

   Queue allows insertion at rear and deletion at front; stack allows insertion and deletion only at one end.

3. Final Answer:

   A data structure where elements can be added or removed from both ends -> Option A

4. Quick Check:

   Deque = double-ended queue [OK]

Hint: Deque means double-ended queue, both ends usable [OK]

Common Mistakes:

• Confusing deque with stack or queue
• Thinking deque allows random access
• Assuming insertion only at one end

2. Which of the following is the correct way to declare a deque using Go's container/list package?

easy

A. var d []int

B. d := list.New()

C. d := make(deque, 0)

D. var d map[int]int

Solution

1. Step 1: Recall Go's deque implementation

   Go does not have a built-in deque type but container/list can be used as a double-ended queue.

2. Step 2: Check syntax for creating a new list

   list.New() returns a pointer to a new doubly linked list, suitable for deque operations.

3. Final Answer:

   d := list.New() -> Option B

4. Quick Check:

   Use list.New() for deque in Go [OK]

Hint: Use list.New() to create a deque in Go [OK]

Common Mistakes:

• Using slice instead of list for deque
• Trying to make a deque type directly
• Confusing map with deque

3. What will be the output of the following Go code using container/list as a deque?

package main
import (
  "container/list"
  "fmt"
)
func main() {
  d := list.New()
  d.PushBack(10)
  d.PushFront(20)
  d.PushBack(30)
  for e := d.Front(); e != nil; e = e.Next() {
    fmt.Print(e.Value, " ")
  }
}

medium

A. 30 10 20

B. 10 20 30

C. 20 10 30

D. 20 30 10

Solution

1. Step 1: Trace the Push operations on deque

   PushBack(10) adds 10 at rear; PushFront(20) adds 20 at front; PushBack(30) adds 30 at rear.

2. Step 2: Traverse from front to back

   Elements order: 20 (front), 10 (middle), 30 (rear).

3. Final Answer:

   20 10 30 -> Option C

4. Quick Check:

   Front to back traversal = 20 10 30 [OK]

Hint: PushFront adds front, PushBack adds rear [OK]

Common Mistakes:

• Mixing order of PushFront and PushBack
• Assuming PushBack adds to front
• Confusing traversal direction

4. Identify the error in this Go code snippet using container/list as a deque:

package main
import (
  "container/list"
  "fmt"
)
func main() {
  d := list.New()
  d.PushFront(1)
  d.Remove(d.Back())
  fmt.Println(d.Back().Value)
}

medium

A. Using PushFront instead of PushBack

B. Not initializing the list with make()

C. Printing d.Back().Value without converting to string

D. Calling Remove on d.Back() before checking if d.Back() is nil

Solution

1. Step 1: Understand Remove and Back usage

   d.Remove(d.Back()) removes the last element; after removal, d.Back() may be nil if list is empty.

2. Step 2: Check the print statement

   fmt.Println(d.Back().Value) accesses Value of d.Back() without checking if d.Back() is nil, causing runtime panic if nil.

3. Final Answer:

   Calling Remove on d.Back() before checking if d.Back() is nil -> Option D

4. Quick Check:

   Check nil before accessing Back() after Remove [OK]

Hint: Check nil after Remove before accessing Back() [OK]

Common Mistakes:

• Ignoring nil check after removal
• Confusing PushFront and PushBack
• Thinking fmt.Println needs string conversion

5. You want to implement a sliding window maximum using a deque in Go. Which deque operation sequence correctly maintains the window elements in decreasing order from front to rear?

// Assume d is a deque of indices
for i := 0; i < len(nums); i++ {
  for d.Len() > 0 && nums[d.Back().Value.(int)] <= nums[i] {
    d.Remove(d.Back())
  }
  d.PushBack(i)
  if d.Front().Value.(int) <= i - k {
    d.Remove(d.Front())
  }
}

hard

A. Remove from back while current element is greater or equal, then add current index at back

B. Remove from front while current element is smaller, then add current index at front

C. Add current index at front without removals

D. Remove from back while current element is smaller, then add current index at front

Solution

1. Step 1: Understand sliding window max logic

   We remove smaller or equal elements from back to keep deque decreasing, then add current index at back.

2. Step 2: Remove indices out of window from front

   If front index is out of current window (i - k), remove it from front to maintain window size.

3. Final Answer:

   Remove from back while current element is greater or equal, then add current index at back -> Option A

4. Quick Check:

   Sliding window max uses back removals and back insertions [OK]

Hint: Remove smaller from back, add new at back for max window [OK]

Common Mistakes:

• Removing from front instead of back for smaller elements
• Adding new elements at front incorrectly
• Not removing out-of-window indices