Remove Nth Node From End of List(no 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 postMark = head;
        ListNode preMark = head;
        
        for (int i = 0; i < n; i++) {
            if (postMark == null) {
                return null;
            }
            postMark = postMark.next;
        }
        
        if (postMark == null) {
            return head.next;
        }
        
        while (postMark.next != null) {
            postMark = postMark.next;
            preMark = preMark.next;
        }
        preMark.next = preMark.next.next;
        
        return head;
    }
}