LeetCode 19. Remove Nth Node From End of List

19. Remove Nth Node From End of List

Difficulty:: Medium

Given a linked list, remove the n-th node from the end of list and return its head.

Example:

Given linked list: 1->2->3->4->5, and n = 2.

After removing the second node from the end, the linked list becomes 1->2->3->5.

Note:

Given n will always be valid.

Follow up:

Could you do this in one pass?

Solution

Language: Java

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *       int val;
 *       ListNode next;
 *       ListNode(int x) { val = x; }
 * }
 */
class Solution {
      public ListNode removeNthFromEnd(ListNode head, int n) {
            if (head == null) {
                  return head;
           }
            
            ListNode slow = head;
            ListNode fast = head;
            for (int i = 0; i < n; i++) {
                  if (fast == null) {
                        return null;
                 }
                  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;
     }
}

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode removeNthFromEnd(ListNode head, int n) {
        if (head == null) {
            return head;
        }
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode node = dummy;
        for (int i = 0; i < n; i++) {
            node = node.next;
        }
        ListNode prev = dummy;
        while (node.next != null) {
            prev = prev.next;
            node = node.next;
        }
        prev.next = prev.next.next;
        return dummy.next;
    }
}