Remove Nth Node from End of List in DSA C# - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main idea behind removing the Nth node from the end of a linked list?

Use two pointers spaced N nodes apart. Move both until the second pointer reaches the end. The first pointer will then be just before the node to remove.

beginner

Why do we use a dummy node at the start of the linked list in this problem?

A dummy node helps handle edge cases like removing the first node easily by providing a fixed node before the head.

intermediate

In the two-pointer approach, what happens if the second pointer reaches the end after moving N steps initially?

It means the node to remove is the head node, so we remove the head by adjusting the dummy's next pointer.

beginner

What is the time complexity of removing the Nth node from the end of a singly linked list using the two-pointer method?

O(L), where L is the length of the list, because we traverse the list at most once.

beginner

What is the role of the first pointer in the two-pointer technique for this problem?

The first pointer points to the node just before the one to remove when the second pointer reaches the end.

What does the dummy node help with when removing the Nth node from the end?

A Speeds up the traversal

B Simplifies removing the head node

C Stores the value of the removed node

D Keeps track of the list length

The dummy node provides a fixed node before the head, making it easier to remove the head node without special cases.

How far apart are the two pointers initially in the two-pointer method?

A Length of list apart

B 1 node apart

C N nodes apart

D They start at the same node

The second pointer moves N steps ahead so that the two pointers are N nodes apart.

If the second pointer reaches the end immediately after moving N steps, which node should be removed?

A The last node

B No node should be removed

C The middle node

D The head node

This means the node to remove is the head node, so we remove it by adjusting the dummy's next pointer.

What is the time complexity of the two-pointer approach for this problem?

A O(L)

B O(N^2)

C O(1)

D O(log L)

We traverse the list at most once, so the time complexity is linear in the length of the list.

What does the first pointer point to when the second pointer reaches the end?

A The node before the one to remove

B The node to remove

C The last node

D The head node

The first pointer is positioned just before the node that needs to be removed.

Explain step-by-step how to remove the Nth node from the end of a singly linked list using the two-pointer technique.

Think about spacing the pointers and how to find the node before the one to delete.

Why is the dummy node important in handling edge cases when removing the Nth node from the end?

Consider what happens if the node to remove is the first node.

Practice

1. What does the problem Remove Nth Node from End of List ask you to do?

easy

A. Delete the node that is the nth from the end in a linked list

B. Add a new node at the nth position from the start

C. Reverse the linked list

D. Find the value of the nth node from the start

Solution

1. Step 1: Understand the problem statement

   The problem asks to remove the node that is nth from the end of a singly linked list.

2. Step 2: Clarify what removal means

   Removal means unlinking that node so the list skips it, keeping the rest intact.

3. Final Answer:

   Delete the node that is the nth from the end in a linked list -> Option A

4. Quick Check:

   Remove nth from end = Delete nth from end [OK]

Hint: Focus on 'remove nth from end' means delete that node [OK]

Common Mistakes:

• Confusing nth from end with nth from start
• Thinking it means reversing the list
• Trying to find value instead of removing node

2. Which of the following C# code snippets correctly declares a singly linked list node class for this problem?

easy

A. public class ListNode { int val; ListNode next; ListNode(int x) { val = x; next = null; } }

B. class Node { int val; Node next; Node(int x) { val = x; next = null; } }

C. public struct ListNode { public int val; public ListNode next; }

D. public class ListNode { public int val; public ListNode next; public ListNode(int x) { val = x; next = null; } }

Solution

1. Step 1: Check for public access modifiers

   public class ListNode { public int val; public ListNode next; public ListNode(int x) { val = x; next = null; } } uses public class and public members, which is correct for external access.

2. Step 2: Verify constructor and member initialization

   public class ListNode { public int val; public ListNode next; public ListNode(int x) { val = x; next = null; } } correctly initializes val and next in constructor; others miss public or use struct incorrectly.

3. Final Answer:

   public class ListNode { public int val; public ListNode next; public ListNode(int x) { val = x; next = null; } } -> Option D

4. Quick Check:

   Proper class with public members = public class ListNode { public int val; public ListNode next; public ListNode(int x) { val = x; next = null; } } [OK]

Hint: Use public class and public members for linked list nodes [OK]

Common Mistakes:

• Missing public keyword causing access errors
• Using struct instead of class for linked list node
• Not initializing next to null in constructor

3. What will be the output linked list after removing the 2nd node from the end in the list: 1 -> 2 -> 3 -> 4 -> 5 -> null?

medium

A. 1 -> 3 -> 4 -> 5 -> null

B. 1 -> 2 -> 4 -> 5 -> null

C. 1 -> 2 -> 3 -> 5 -> null

D. 2 -> 3 -> 4 -> 5 -> null

Solution

1. Step 1: Identify the 2nd node from the end

   The list is 1->2->3->4->5. Counting from end: 5 (1st), 4 (2nd). So node with value 4 is to be removed.

2. Step 2: Remove node with value 4

   Link node 3's next to node 5, skipping node 4. Result: 1->2->3->5->null.

3. Final Answer:

   1 -> 2 -> 3 -> 5 -> null -> Option C

4. Quick Check:

   Remove 2nd from end (4) = 1->2->3->5 [OK]

Hint: Count from end to find node, then skip it [OK]

Common Mistakes:

• Removing the wrong node counting from start
• Removing node with value 3 instead of 4
• Not updating links correctly causing broken list

4. Given the following code snippet to remove the nth node from end, what is the bug?

public ListNode RemoveNthFromEnd(ListNode head, int n) {
  ListNode fast = head;
  ListNode slow = head;
  for (int i = 0; i < n; i++) {
    fast = fast.next;
  }
  while (fast.next != null) {
    fast = fast.next;
    slow = slow.next;
  }
  slow.next = slow.next.next;
  return head;
}

medium

A. Does not handle case when fast becomes null after initial loop

B. Uses slow pointer incorrectly by moving it too early

C. Returns slow instead of head

D. Does not update fast pointer inside the loop

Solution

1. Step 1: Analyze fast pointer movement

   Fast moves n steps ahead. If n equals list length, fast becomes null.

2. Step 2: Check for null fast before accessing fast.next

   Code does not check if fast is null before while loop, causing NullReferenceException.

3. Final Answer:

   Does not handle case when fast becomes null after initial loop -> Option A

4. Quick Check:

   Null fast after n steps = bug [OK]

Hint: Check if fast is null after initial n steps [OK]

Common Mistakes:

• Ignoring null check for fast pointer
• Moving slow pointer before fast is properly positioned
• Returning wrong node as new head

5. You want to remove the 1st node from the end of a linked list with only one node: 10 -> null. What should the resulting list be?

hard

A. 10 -> null

B. null (empty list)

C. Error because list is too short

D. 10 -> 10 -> null

Solution

1. Step 1: Understand removing 1st node from end in single-node list

   Removing last node means list becomes empty.

2. Step 2: Resulting list after removal

   Since only node is removed, list is now null (empty).

3. Final Answer:

   null (empty list) -> Option B

4. Quick Check:

   Remove only node = empty list [OK]

Hint: Removing last node in single-node list empties it [OK]

Common Mistakes:

• Returning original list instead of empty
• Expecting error instead of empty list
• Trying to access next node which does not exist