19. 删除链表的倒数第 N 个结点

/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} head
 * @param {number} n
 * @return {ListNode}
 */
 var removeNthFromEnd = function(head, n) {
                let pointFront,pointBack ;
                let flag=0,reArray = new ListNode();
                head.front = null;
                pointFront = pointBack = head;

                let count = 1;
                //  使单链表变成双向链表
                while(pointBack.next!= null){// pointFront始终指向的是前一个节点 pointBack始终指向的是后一个节点
                    pointBack = pointFront.next;
                    pointBack.front = pointFront;
                    pointFront = pointBack;
                    flag = 1;
                 }
                 if(flag==0) return head.next;
                pointFront = pointBack.front;
                // 找到最后一个结点，现在开始往回找
                while(count < n){//小于说明还没有找到
                    pointBack = pointFront;
                    pointFront = pointFront.front;
                    count++;
                     
                }
                if(pointBack.front == null) 
                    return pointBack.next;
                else
                    pointFront.next = pointBack.next;
                return head;
 };