Remove Nth Node From End of List(dummy).java

/**
 * Definition for ListNode.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int val) {
 *         this.val = val;
 *         this.next = null;
 *     }
 * }
 */ 
public class Solution {
    /**
     * @param head: The first node of linked list.
     * @param n: An integer.
     * @return: The head of linked list.
     */
    ListNode removeNthFromEnd(ListNode head, int n) {
        // write your code here
        if (head == null || n < 0) {
            return null;
        }
        
        ListNode dummyHead = new ListNode(0);
        dummyHead.next = head;
        
        ListNode preHead = dummyHead;
        
        for (int i = 0; i < n; i++) {
            if (head == null) {
                return null;
            }
            head = head.next;
        }
        
        while (head != null) {
            head = head.next;
            preHead = preHead.next;
        }
        preHead.next = preHead.next.next;
        
        return dummyHead.next;
    }
}