LeetCode19. 删除链表的倒数第N个节点

思路：同剑指14.链表中倒数第k个结点

class Solution {
    public ListNode removeNthFromEnd(ListNode head, int n) {
        /**
         *  方法1：计算链表长度，相当于两次遍历链表
         */
        /*
        ListNode dummyHead = new ListNode(-1);
        dummyHead.next = head;
        int len = 0;
        ListNode cur = dummyHead;
        while (cur != null) {
            len ++;
            cur = cur.next;
        }
        cur = dummyHead;
        for (int i = 0; i < len - n - 1; i++) {
            cur = cur.next;
        }
        cur.next = cur.next.next;
        return dummyHead.next;
        */
        /**
         * 方法2： 快慢指针。 一次遍历
         */
        ListNode dummyHead = new ListNode(-1);
        dummyHead.next = head;
        ListNode fast = dummyHead, slow = dummyHead;
        for (int i = 0; i < n + 1; i++) {
            fast = fast.next;
        }
        while (fast != null) {
            fast = fast.next;
            slow = slow.next;
        }
        slow.next = slow.next.next;
        return dummyHead.next;
    }
}