Pop Using Linked List Node in DSA Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the 'pop' operation do in a linked list?

It removes the first node (head) from the linked list and returns its value.

beginner

In a singly linked list, what pointer is updated when you pop a node?

The head pointer is updated to point to the next node after the removed node.

beginner

Why must you check if the linked list is empty before popping?

Because popping from an empty list can cause errors or exceptions since there is no node to remove.

intermediate

What is the time complexity of the pop operation in a singly linked list?

O(1) because it removes the head node directly without traversing the list.

intermediate

Show the basic Java code snippet to pop a node from a singly linked list.

if (head == null) return null; Node poppedNode = head; head = head.next; poppedNode.next = null; return poppedNode;

What happens to the head pointer after popping a node from a linked list?

A It points to the next node after the popped node

B It points to the popped node

C It becomes null always

D It points to the last node

After popping, the head pointer moves to the next node in the list.

What should you do before popping a node from a linked list?

A Reverse the list

B Check if the list is empty

C Traverse the entire list

D Sort the list

You must check if the list is empty to avoid errors when popping.

What is the time complexity of popping a node from the head of a singly linked list?

A O(n)

B O(log n)

C O(n^2)

D O(1)

Popping from the head is a constant time operation, O(1).

Which node is removed during the pop operation in a singly linked list?

A The last node

B The middle node

C The head node

D A random node

Pop removes the head node from the list.

After popping, what should you do with the removed node's next pointer?

A Set it to null

B Point it to head

C Leave it unchanged

D Point it to tail

Setting the popped node's next pointer to null helps avoid accidental links.

Explain step-by-step how to pop a node from a singly linked list.

Think about what happens to the head pointer and the removed node.

Why is popping from a linked list efficient compared to other operations?

Consider how many nodes you need to visit to pop.

Practice

1. What does the pop() operation do in a linked list when implemented using nodes?

easy

A. Removes and returns the first node of the linked list

B. Adds a new node at the end of the linked list

C. Removes the last node without returning it

D. Returns the size of the linked list

Solution

1. Step 1: Understand pop operation in linked list context

   The pop operation removes the top element, which is the first node in a linked list acting like a stack.

2. Step 2: Identify what is returned

   Pop returns the value of the removed first node and updates the head pointer to the next node.

3. Final Answer:

   Removes and returns the first node of the linked list -> Option A

4. Quick Check:

   Pop = Remove first node [OK]

Hint: Pop removes the head node and returns it [OK]

Common Mistakes:

• Thinking pop removes the last node
• Confusing pop with push operation
• Assuming pop returns list size

2. Which of the following is the correct Java syntax to pop a node from the head of a singly linked list?

easy

A. head = head.next; return head.data;

B. Node temp = head; head = head.next; return temp.data;

C. Node temp = head.next; head = temp; return head.data;

D. return head.next.data; head = head.next;

Solution

1. Step 1: Check correct order of operations

   We must save the current head node in a temp variable before moving head to next.

2. Step 2: Return the data of the removed node

   Return the data from the saved temp node, not the new head.

3. Final Answer:

   Node temp = head; head = head.next; return temp.data; -> Option B

4. Quick Check:

   Save head, move head, return old data [OK]

Hint: Save head before moving it to next [OK]

Common Mistakes:

• Returning data from new head instead of old
• Not saving old head before moving
• Returning data before updating head

3. Given the linked list nodes: 10 -> 20 -> 30 -> null, what is the list after one pop operation using this code?

Node pop() {
  if (head == null) return null;
  Node temp = head;
  head = head.next;
  return temp;
}

medium

A. 30 -> null

B. 10 -> 20 -> 30 -> null

C. 20 -> 30 -> null

D. null

Solution

1. Step 1: Identify the node removed by pop()

   Pop removes the head node which has value 10.

2. Step 2: Update head to next node

   Head moves from 10 to 20, so list becomes 20 -> 30 -> null.

3. Final Answer:

   20 -> 30 -> null -> Option C

4. Quick Check:

   Pop removes first node 10 [OK]

Hint: Pop removes first node, head moves next [OK]

Common Mistakes:

• Assuming pop removes last node
• Not updating head pointer
• Returning old list unchanged

4. What is the error in this pop method for a singly linked list?

Node pop() {
  if (head == null) return null;
  Node temp = head.next;
  head = temp;
  return temp;
}

medium

A. It skips the first node and removes the second node instead

B. It causes a null pointer exception when list is empty

C. It returns the old head node instead of new head

D. It does not update the head pointer

Solution

1. Step 1: Analyze which node is assigned to temp

   Temp is assigned head.next, so the first node is skipped.

2. Step 2: Understand effect on head and return

   Head moves to second node, so first node is never removed; second node is removed instead.

3. Final Answer:

   It skips the first node and removes the second node instead -> Option A

4. Quick Check:

   Pop must remove first node, not second [OK]

Hint: Pop must save head, not head.next [OK]

Common Mistakes:

• Assigning temp to head.next instead of head
• Returning the new head instead of the old head
• Returning wrong node

5. You have a linked list: 5 -> 15 -> 25 -> 35 -> null. You perform two pop operations using this method:

Node pop() {
  if (head == null) return null;
  Node temp = head;
  head = head.next;
  return temp;
}

What is the state of the list after these two pops?

hard

A. null

B. 15 -> 25 -> 35 -> null

C. 35 -> null

D. 25 -> 35 -> null

Solution

1. Step 1: First pop removes 5

   After first pop, head moves from 5 to 15, list is 15 -> 25 -> 35 -> null.

2. Step 2: Second pop removes 15

   After second pop, head moves from 15 to 25, list is 25 -> 35 -> null.

3. Final Answer:

   25 -> 35 -> null -> Option D

4. Quick Check:

   Two pops remove first two nodes [OK]

Hint: Each pop removes head, moves head next [OK]

Common Mistakes:

• Removing last nodes instead of first
• Not updating head after each pop
• Confusing pop with push