Double Ended Queue Deque in DSA C - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Deque Operations Sequence

What is the printed state of the deque after performing these operations?

Start with an empty deque.
Operations:
1. Add 10 to front
2. Add 20 to rear
3. Add 30 to front
4. Remove from rear
5. Add 40 to rear

DSA C

Deque deque = createDeque();
addFront(&deque, 10);
addRear(&deque, 20);
addFront(&deque, 30);
removeRear(&deque);
addRear(&deque, 40);
printDeque(&deque);

A 10 -> 20 -> 40 -> null

B 10 -> 30 -> 40 -> null

C 30 -> 20 -> 40 -> null

D 30 -> 10 -> 40 -> null

💡 Hint

Remember that addFront adds to the front and addRear adds to the end. Removing from rear removes the last element.

Step-by-step:
1. addFront 10: deque = 10
2. addRear 20: deque = 10 -> 20
3. addFront 30: deque = 30 -> 10 -> 20
4. removeRear: removes 20, deque = 30 -> 10
5. addRear 40: deque = 30 -> 10 -> 40

🧠 Conceptual

intermediate

Deque Behavior on Mixed Operations

If you perform these operations on an empty deque:
addRear(5), addRear(10), addFront(3), removeFront(), removeRear()
What is the final state of the deque?

A 5 -> null

B 3 -> 5 -> 10 -> null

C 10 -> null

D 3 -> null

💡 Hint

Track each operation carefully and update the deque after each step.

Operations:
1. addRear(5): deque = 5
2. addRear(10): deque = 5 -> 10
3. addFront(3): deque = 3 -> 5 -> 10
4. removeFront(): removes 3, deque = 5 -> 10
5. removeRear(): removes 10, deque = 5

🔧 Debug

advanced

Identify the Error in Deque Implementation

Given this snippet of C code for removing from the rear of a deque, what error will occur?

```c void removeRear(Deque* dq) { if (dq->rear == NULL) return; Node* temp = dq->rear; dq->rear = dq->rear->prev; if (dq->rear != NULL) dq->rear->next = NULL; free(temp); } ```

DSA C

void removeRear(Deque* dq) {
  if (dq->rear == NULL) return;
  Node* temp = dq->rear;
  dq->rear = dq->rear->prev;
  if (dq->rear != NULL) dq->rear->next = NULL;
  free(temp);
}

A Memory leak due to not freeing temp

B Dereferencing NULL pointer if deque has only one element

C No error; code works correctly

D Double free error on temp

💡 Hint

Check what happens when deque has one element and how pointers update.

If deque has one element, dq->rear->prev is NULL, so dq->rear becomes NULL. The if condition prevents dereferencing NULL. Memory is freed properly. No error occurs.

❓ Predict Output

advanced

Deque State After Complex Operations

What is the deque state after these operations?

Start empty.
addRear(1), addRear(2), addFront(3), removeFront(), addRear(4), removeRear(), addFront(5)

DSA C

Deque dq = createDeque();
addRear(&dq, 1);
addRear(&dq, 2);
addFront(&dq, 3);
removeFront(&dq);
addRear(&dq, 4);
removeRear(&dq);
addFront(&dq, 5);
printDeque(&dq);

A 5 -> 1 -> null

B 5 -> 1 -> 2 -> null

C 1 -> 2 -> 4 -> null

D 3 -> 1 -> 4 -> null

💡 Hint

Track each operation carefully and update the deque after each step.

Stepwise:
addRear(1): 1
addRear(2): 1 -> 2
addFront(3): 3 -> 1 -> 2
removeFront(): removes 3, deque = 1 -> 2
addRear(4): 1 -> 2 -> 4
removeRear(): removes 4, deque = 1 -> 2
addFront(5): 5 -> 1 -> 2

🧠 Conceptual

expert

Maximum Number of Elements in Circular Deque

A circular deque is implemented using an array of size 6. If the deque is full, how many elements can it hold at most?

A 5

B 6

C 4

D 7

💡 Hint

In circular deque implementations, one slot is usually kept empty to distinguish full and empty states.

For an array of size n, maximum elements stored is n-1 to differentiate full from empty. Here, 6 - 1 = 5.