19. 删除链表的倒数第 N 个结点

链表常见解法：
使用前后指针, 前指针遍历完时后指针指向删除节点；

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode removeNthFromEnd(ListNode head, int n) {
        ListNode dummy=new ListNode(0,head);
        ListNode back  = dummy;
        ListNode front = head;
        for(int i=0;i<n;i++){
            front=front.next;
        }
        while(front!=null){
            front=front.next;
            back=back.next;
        }
        back.next=back.next.next;
        return dummy.next;
    }
}