Remove Nth Node from End of List in DSA Javascript - 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 before moving N steps?

It means the list has fewer than N nodes, so no removal is possible or the head needs to be removed if exactly N nodes.

beginner

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

O(L), where L is the length of the list, because we traverse the list at most twice with two pointers.

beginner

How do you update the pointers to remove the target node once identified?

Change the next pointer of the node before the target to skip the target node, linking it to the node after the target.

What is the purpose of the two-pointer technique in this problem?

A To sort the linked list

B To reverse the linked list

C To count the total nodes in the list

D To find the node before the Nth node from the end

The two-pointer technique helps locate the node just before the one to remove by spacing pointers N nodes apart.

Why is a dummy node often added before the head of the list?

A To increase the list size

B To simplify removal of the head node

C To store the value of N

D To reverse the list

A dummy node simplifies edge cases, especially when the head node itself needs to be removed.

If N equals the length of the list, which node is removed?

A The head node

B The last node

C The middle node

D No node is removed

When N equals the list length, the Nth node from the end is the head node.

What is the time complexity of this removal operation?

A O(N^2)

B O(1)

C O(L)

D O(log L)

The operation requires traversing the list once or twice, so it is linear in the list length.

After locating the node before the target, how do you remove the target node?

A Set its next pointer to skip the target node

B Delete the entire list

C Reverse the list

D Swap values with the target node

You remove the target by linking the previous node's next pointer to the target's next node.

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

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

What edge cases should you consider when removing the Nth node from the end of a list?

Consider what happens when the node to remove is at the start or when N is large.

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 up to the nth node

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

Solution

1. Step 1: Understand the problem statement

   The problem asks to remove a node counting from the end of a linked list, not from the start.

2. Step 2: Identify the correct action

   Removing the nth node from the end means deleting that specific node and linking its previous node to the next node.

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: Count nodes from end, not start, to remove correct node [OK]

Common Mistakes:

• Confusing nth from end with nth from start
• Trying to reverse the list instead of removing
• Removing the wrong node index

2. Which of the following is the correct way to declare a function in JavaScript to remove the nth node from the end of a singly linked list?

easy

A. function removeNthFromEnd(head, n) { /* code */ }

B. removeNthFromEnd = (head, n) => { /* code */ }

C. All of the above

D. const removeNthFromEnd = function(head, n) { /* code */ }

Solution

1. Step 1: Review JavaScript function declaration styles

   JavaScript supports function declarations, function expressions, and arrow functions.

2. Step 2: Check each option

   function removeNthFromEnd(head, n) { /* code */ } is a function declaration, B is an arrow function, C is a function expression. All are valid ways to declare functions.

3. Final Answer:

   All of the above -> Option C

4. Quick Check:

   All function styles valid in JS [OK]

Hint: JS allows multiple function declaration styles [OK]

Common Mistakes:

• Thinking only one function syntax is valid
• Confusing arrow functions with older syntax
• Missing const or function keyword

3. What will be the output linked list after running this code snippet?

class ListNode {
  constructor(val, next = null) {
    this.val = val;
    this.next = next;
  }
}

function removeNthFromEnd(head, n) {
  let dummy = new ListNode(0, head);
  let first = dummy;
  let second = dummy;
  for (let i = 0; i <= n; i++) {
    first = first.next;
  }
  while (first !== null) {
    first = first.next;
    second = second.next;
  }
  second.next = second.next.next;
  return dummy.next;
}

// List: 1 -> 2 -> 3 -> 4 -> 5 -> null
let head = new ListNode(1, new ListNode(2, new ListNode(3, new ListNode(4, new ListNode(5)))));
let newHead = removeNthFromEnd(head, 2);

// Print list
let result = [];
while (newHead) {
  result.push(newHead.val);
  newHead = newHead.next;
}
console.log(result);

medium

A. [1, 2, 3, 4]

B. [1, 2, 4, 5]

C. [2, 3, 4, 5]

D. [1, 2, 3, 5]

Solution

1. Step 1: Understand the code logic

   The code uses two pointers with a dummy node to remove the nth node from the end. Here, n=2 means remove the 2nd last node (value 4).

2. Step 2: Trace the linked list after removal

   Original list: 1 -> 2 -> 3 -> 4 -> 5. Removing node with value 4 results in 1 -> 2 -> 3 -> 5.

3. Final Answer:

   [1, 2, 3, 5] -> Option D

4. Quick Check:

   Remove 2nd last node = remove 4 [OK]

Hint: Use two pointers spaced n apart to find node to remove [OK]

Common Mistakes:

• Removing the wrong node counting from start
• Not using dummy node causing errors on head removal
• Off-by-one error in loop for pointer movement

4. Identify the bug in this code snippet for removing the nth node from the end of a linked list:

function removeNthFromEnd(head, n) {
  let dummy = new ListNode(0, head);
  let first = dummy;
  let second = dummy;
  for (let i = 0; i < n; i++) {
    first = first.next;
  }
  while (first.next !== null) {
    first = first.next;
    second = second.next;
  }
  second.next = second.next.next;
  return dummy.next;
}

medium

A. The while loop condition should be first !== null instead of first.next !== null

B. The for loop should run from i = 0 to i <= n to move first pointer correctly

C. The dummy node is not needed and causes errors

D. second.next = second.next.next should be second.next.next = second.next

Solution

1. Step 1: Analyze the for loop pointer movement

   The for loop moves the first pointer n times, which is correct to create a gap of n nodes between first and second pointers.

2. Step 2: Understand the while loop condition

   The while loop condition should be first !== null to ensure the first pointer reaches the end, allowing the second pointer to point to the node before the target.

3. Final Answer:

   The while loop condition should be first !== null instead of first.next !== null -> Option A

4. Quick Check:

   Use while (first !== null) to correctly position second pointer [OK]

Hint: Use while (first !== null) to move pointers correctly [OK]

Common Mistakes:

• Moving first pointer only n times instead of n+1
• Incorrect while loop condition causing null errors
• Removing dummy node mistakenly

5. Given a linked list 1 -> 2 -> 3 -> 4 -> 5 -> null, what will be the resulting list after removing the 5th node from the end using the standard two-pointer approach?

hard

A. [1, 2, 3, 4]

B. [2, 3, 4, 5]

C. [1, 3, 4, 5]

D. [1, 2, 3, 5]

Solution

1. Step 1: Identify the node to remove

   Removing the 5th node from the end means removing the first node (value 1) because the list length is 5.

2. Step 2: Remove the head node and link next node as new head

   After removal, the list starts from node with value 2: 2 -> 3 -> 4 -> 5.

3. Final Answer:

   [2, 3, 4, 5] -> Option B

4. Quick Check:

   Remove 5th from end = remove head [OK]

Hint: Removing nth from end where n = list length removes head [OK]

Common Mistakes:

• Not handling removal of head node correctly
• Returning original list without removal
• Confusing nth from end with nth from start