leetcode 19 删除链表的倒数第N个节点

法一 单指针（两次遍历）

class Solution {
    public ListNode removeNthFromEnd(ListNode head, int n) {
        if(head == null)
            return null;
        int L = 0;
        ListNode dummyHead = new ListNode(-1);
        dummyHead.next = head;
        ListNode prev = dummyHead;
        while(prev.next != null){
            L ++;
            prev = prev.next;
        }
        prev = dummyHead;
        for(int i = L - n; i > 0; i --){
            prev = prev.next;
        }
        ListNode del = prev.next;
        prev.next = del.next;
        del.next = null;
        return dummyHead.next;
    }
}

法二 双指针（一次遍历）

class Solution {
    public ListNode removeNthFromEnd(ListNode head, int n) {
        ListNode fast = head;
        while(n  > 0){
            n --;
            fast = fast.next;
        }
        if(fast == null)
            return head.next;
        ListNode slow = head;
        while(fast.next != null){
            fast = fast.next;
            slow = slow.next;
        }
        slow.next = slow.next.next;
        return head;
    }
}