19. Remove Nth Node From End of List

Total Accepted: 96063
Total Submissions: 333812
Difficulty: Easy

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

For 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.
Try to do this in one pass.

Java:

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode removeNthFromEnd(ListNode head, int n) {
        ListNode pre = head, after = head;
        while (n-- != 0) {
            after = after.next;
        }
        if (after != null) {
            while (after.next != null) {
                pre = pre.next;
                after = after.next;
            }
            pre.next = pre.next.next;
        } else {
            return head.next;
        }
        return head;
    }
}

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