Remove Nth Node from End of List in DSA Javascript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize a dummy node pointing to the head.

DSA Javascript

const dummy = new ListNode(0);
dummy.next = [1];

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

Ahead

Bnull

Cdummy

Dundefined

Attempts:

3 left

💡 Hint

Common Mistakes

Setting dummy.next to null breaks the list.

Using dummy instead of head causes infinite loops.

We create a dummy node and set its next pointer to the head of the list to simplify edge cases.

2fill in blank

medium

Complete the code to move the fast pointer n steps ahead.

DSA Javascript

for (let i = 0; i < n; i++) {
  fast = fast[1];
}

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

A[i]

B.prev

C.next

D++

Attempts:

3 left

💡 Hint

Common Mistakes

Using .prev instead of .next moves backward or causes errors.

Using ++ on a pointer is invalid.

To move forward in a linked list, we use the .next pointer.

3fill in blank

hard

Fix the error in the while loop condition to move both pointers until fast reaches the end.

DSA Javascript

while (fast[1] !== null) {
  fast = fast.next;
  slow = slow.next;
}

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

Anext

B[next]

C->next

D.next

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'next' without dot causes syntax errors.

Using arrow or bracket notation is invalid in JavaScript.

In JavaScript, to access a property, use dot notation without quotes: .next

4fill in blank

hard

Fill both blanks to remove the nth node from the end by skipping it.

DSA Javascript

slow[1] = slow[2].next;

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

A.next

B.prev

D.next.next

Attempts:

3 left

💡 Hint

Common Mistakes

Using .prev instead of .next causes errors.

Not skipping the node properly leaves the list unchanged.

We set slow.next to slow.next.next to skip the node to be removed.

5fill in blank

hard

Complete the code to return the new head of the list after removal.

DSA Javascript

return dummy[1];

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

A.next

B.prev

C.next.next

Dhead

Attempts:

3 left

💡 Hint

Common Mistakes

Returning head may be incorrect if the first node was removed.

Using .prev or .next.next on dummy is invalid.

After removal, the new head is dummy.next, which may differ if the first node was removed.

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