19. Remove Nth Node From End of List ¶

Time: $O(n)$
Space: $O(1)$

C++JavaPythonJavaScript

class Solution {
 public:
  ListNode* removeNthFromEnd(ListNode* head, int n) {
    ListNode* slow = head;
    ListNode* fast = head;

    while (n-- > 0)
      fast = fast->next;
    if (fast == nullptr)
      return head->next;

    while (fast->next != nullptr) {
      slow = slow->next;
      fast = fast->next;
    }
    slow->next = slow->next->next;

    return head;
  }
};

class Solution {
  public ListNode removeNthFromEnd(ListNode head, int n) {
    ListNode slow = head;
    ListNode fast = head;

    while (n-- > 0)
      fast = fast.next;
    if (fast == null)
      return head.next;

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

    return head;
  }
}

class Solution:
  def removeNthFromEnd(self, head: ListNode, n: int) -> ListNode:
    slow = head
    fast = head

    for _ in range(n):
      fast = fast.next
    if not fast:
      return head.next

    while fast.next:
      slow = slow.next
      fast = fast.next
    slow.next = slow.next.next

    return head

var removeNthFromEnd = function(head, n) {
  const dummy = new ListNode(0, head);
  let fast = dummy, slow = dummy;
  for (let i = 0; i <= n; i++) fast = fast.next;
  while (fast) { fast = fast.next; slow = slow.next; }
  slow.next = slow.next.next;
  return dummy.next;
};