Leetcode題解 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.

For example,

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

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

根據常量時間刪除某個節點的做法，看要刪除的節點是否在最後一個分兩種情況討論。

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode removeNthFromEnd(ListNode head, int n) {
        if(null==head||n<=0) return null;
        else if(head.next==null&&n==1) return null;
        if(n!=1){
            ListNode fast=head;
            ListNode slow=head;
            while(fast!=null&&n>1){
                fast=fast.next;
                n--;
            }
            if(n>1||fast==null){
                return null;
            }
            while(fast.next!=null){
                fast=fast.next;
                slow=slow.next;
            }
            slow.val=slow.next.val;
            slow.next=slow.next.next;
            return head;
        }else{
            ListNode temp=head;
            ListNode pre=null;
            while(temp.next!=null){
                pre=temp;
                temp=temp.next;
            }
            pre.next=null;
            return head;
        }

    }
}