LeetCode Q19 Remove Nth Node From End of List

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Symbols count in article: 797 Reading time ≈ 1 mins. 
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/**
 * 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)
    {
        //1. create a dummyHead before the real head
        ListNode dummyHead = new ListNode(0);
        dummyHead.next = head;
        ListNode first = head;
        int length = 0;
        
        //as long as first is not null, we increase length
        //and keep moving first
        while(first != null)
        {
            length++;
            first = first.next;
        }
        //use length - n to locate the node we are going to delete
        length -=n;
        first = dummyHead;
        while(length > 0)
        {
            length--;
            first = first.next;
        }
        first.next = first.next.next;
        return dummyHead.next;
    }
}