Java for LeetCode 143 Reorder List

Given a singly linked list L: L0L1→…→Ln-1Ln,
reorder it to: L0LnL1Ln-1L2Ln-2→…

You must do this in-place without altering the nodes' values.

For example,
Given {1,2,3,4}, reorder it to {1,4,2,3}.

解题思路一:

每次将Ln换到前面,得到L0LnL1L2L3→,然后对L1使用相同操作,JAVA实现如下:

	public void reorderList(ListNode head) {
		ListNode headCopy = head;
		while (headCopy != null && headCopy.next != null
				&& headCopy.next.next != null) {
			ListNode temp = headCopy;
			while (temp.next.next != null)
				temp = temp.next;
			temp.next.next = headCopy.next;
			headCopy.next = temp.next;
			temp.next = null;
			temp = headCopy.next.next;
			headCopy=headCopy.next.next;
		}
	}

 结果TLE

解题思路二:

空间换时间,将所有的node存到一个list中,然后每次操作list头尾两个node即可,JAVA实现如下:

    public void reorderList(ListNode head) {
		LinkedList<ListNode> list = new LinkedList<ListNode>();
		ListNode headCopy = head,end = head;
		while (headCopy != null) {
			list.add(headCopy);
			headCopy = headCopy.next;
		}
		while(list.size()>2){
			headCopy=list.poll();
			end=list.get(list.size()-1);
			list.remove(list.size()-1);
			headCopy.next=end;
			end.next=list.peek();
			list.get(list.size()-1).next=null;
		}
    }

 

posted @ 2015-06-04 20:53  TonyLuis  阅读(277)  评论(0编辑  收藏  举报