查找和删除倒数第n个节点的问题

 class ListNode
 {
     int val;
     ListNode next;
     ListNode(int x)
     {
         val = x;
     }
 }
 public class NthNodeFromEnd {
      public ListNode removeNthFromEnd(ListNode head, int n)
      {
          ListNode p = head;
          ListNode q = head;
          for(int i = 0; i < n ; i++)
          {
              p = p.next;
          }
          if(p == null)//如果p==null，说明倒数第n个就是第一个节点，即头结点。
          {
              head = head.next;
              return head;
          }
          while(p.next != null)
          {
              p = p.next;
              q = q.next;
          }
          if(q.next != null)
          {
              q.next = q.next.next;
          }

          return head;
      }
      public ListNode removeNthFromEnd2(ListNode head, int n)
      {
         //因为要删掉倒数第n个节点，那就必须找到倒数第n+1个节点
         //如果倒数第n正好是头结点，那么头结点前面没有节点了很麻烦，我们可以设置一个节点作为头结点之前的节点
         //设置一个头结点，这样就不用单独对原始头结点处理了
          ListNode dumy = new ListNode(0);
          dumy.next = head;
          ListNode p = dumy;
          ListNode q = dumy;
          for(int i = 0; i < n; i ++)
          {
              p = p.next;
          }
          while(p.next!=null)
          {
              p = p.next;
              q = q.next;
          }
          q.next = q.next.next;
          return dumy.next;
      }
      //找倒数第n个节点
      public ListNode findNthFromEnd(ListNode head, int n)
      {
          ListNode p = head;
          ListNode q = head;
          for(int i = 0; i < n-1; i ++)
          {
              p = p.next;
          }
          while(p.next!=null)
          {
              p = p.next;
              q = q.next;
          }
          return q;
      }

      public static void main(String[] args)
      {
          NthNodeFromEnd rn = new NthNodeFromEnd();
          ListNode n1 = new ListNode(1);
          ListNode n = rn.removeNthFromEnd(n1, 1);
          System.out.println(rn.findNthFromEnd(n1, 1).val);
          if(n == null)
          {
              System.out.println(n);
          }
          while(n!= null)
          {
              System.out.println(n.val);
              n = n.next;
          }
      }
 }