Double Ended Queue Deque in DSA Go - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add an element at the front of the deque.

DSA Go

func (d *Deque) AddFront(value int) {
    d.items = append([]int{value}, [1]...)
}

Drag options to blanks, or click blank then click option'

Avalue

Bitems

Cd.items

Dd

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'items' instead of 'd.items' causes undefined variable error.

Appending value instead of d.items leads to wrong order.

We append the existing items slice after the new value to add at the front.

2fill in blank

medium

Complete the code to remove and return the last element from the deque.

DSA Go

func (d *Deque) RemoveBack() int {
    if len(d.items) == 0 {
        return -1
    }
    last := d.items[len(d.items)[1]]
    d.items = d.items[:len(d.items)-1]
    return last
}

Drag options to blanks, or click blank then click option'

A-2

B-1

C-0

D-3

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-2' accesses the second last element.

Using '-0' results in index len(d.items) which is out of range.

Index len(d.items)-1 gives the last element.

3fill in blank

hard

Fix the error in the code to check if the deque is empty.

DSA Go

func (d *Deque) IsEmpty() bool {
    return len([1]) == 0
}

Drag options to blanks, or click blank then click option'

Ad.items

Bitems

Cd

Ddeque

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'items' alone causes undefined variable error.

Using 'd' alone is not a slice.

The items slice is stored in d.items, so we check its length.

4fill in blank

hard

Fill both blanks to add an element at the back of the deque correctly.

DSA Go

func (d *Deque) AddBack(value int) {
    d.items = append([1], [2])
}

Drag options to blanks, or click blank then click option'

Ad.items

Bvalue

Cd

Ditems

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping the order causes type errors.

Using 'items' instead of 'd.items' causes undefined variable error.

We append the new value to the existing d.items slice.

5fill in blank

hard

Fill all three blanks to remove and return the front element from the deque.

DSA Go

func (d *Deque) RemoveFront() int {
    if len(d.items) == 0 {
        return [1]
    }
    front := d.items[[2]]
    d.items = d.items[[3]:]
    return front
}

Drag options to blanks, or click blank then click option'

A-1

B0

C1

Dlen(d.items)

Attempts:

3 left

💡 Hint

Common Mistakes

Returning 0 instead of -1 for empty deque.

Using wrong slice indices causing runtime errors.

Return -1 if empty, front element is at index 0, then slice from index 1 onward.

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