19-删除链表的倒数第n个节点

力扣19

https://leetcode-cn.com/problems/remove-nth-node-from-end-of-list/

双指针经典题

前后指针，一遍扫描
前面的指针先走n个，然后前后指针一起移动，在要删除的节点前面停下，也就是pre.next != null
然后删掉第n个：post.next = post.next.next

public ListNode removeNthFromEnd2(ListNode head, int n) {
    if (head == null) return null;
    ListNode root = new ListNode(0);
    root.next = head;

    ListNode pre = root, post = root;
    while (n-- != 0) {
        pre = pre.next;
    }

    while (pre != null && pre.next != null){
        pre = pre.next;
        post = post.next;
    }
    if (post != null && post.next != null) post.next = post.next.next;
    return root.next;
}