19. Remove Nth Node From End of List

Given a linked list, remove the nth node from the end of list and return its head.

 Notice

The minimum number of nodes in list is n.

Example

Given linked list: 1->2->3->4->5->null, and n = 2.

After removing the second node from the end, the linked list becomes 1->2->3->5->null.

Challenge 

Can you do it without getting the length of the linked list?

分析：

为了删除倒数第n个node,我们需要找到那个node的parent，然后parent.next = parent.next.next. 这里我们创建一个dummy node用来处理删除的是第一个node的情况。

public class Solution {
    public ListNode removeNthFromEnd(ListNode head, int n) {
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        
        ListNode parent = dummy;
        for (int i = 0; i < n; i++) {
            if (head == null) {
                return null;
            }
            head = head.next;
        }
        while (head != null) {
            head = head.next;
            parent = parent.next;
        }
        parent.next = parent.next.next;
        return dummy.next;
    }
}

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