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    1. // 快慢指针
    2. public ListNode removeNthFromEnd(ListNode head, int n) {
    3.     ListNode fast = head;
    4.     ListNode slow = head;
    5.     for (int i = 0;i < n; i++) {
    6.         fast = fast.next;
    7.     }
    8.     if (fast == null) {
    9.         head = head.next;
    10.         return head;
    11.     }
    12.     while (fast.next != null) {
    13.         fast = fast.next;
    14.         slow = slow.next;
    15.     }
    16.     slow.next = slow.next.next;
    17.     return head;
    18. }
    20. // 递归版本
    21. class Solution {
    22.     public ListNode removeNthFromEnd(ListNode head, int n) {
    23.         return remove(head,n) == n ? head.next : head;
    24.     }
    26.     public int remove (ListNode head, int n) {
    27.         if (head.next == null) {
    28.             return 1;
    29.         }
    30.         int count = remove(head.next,n);
    31.         if (count == n) {
    32.             if (count == 1) {
    33.                 head.next = null;
    34.             } else {
    35.                 head.next = head.next.next;
    36.             }
    37.         }
    38.         return count + 1;
    39.     }
    40. }