Java for LeetCode 143 Reorder List

Given a singly linked list L: L₀→L₁→…→L_n-1→L_n,
reorder it to: L₀→L_n→L₁→L_n-1→L₂→L_n-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L₀→L_n→L₁→L₂→L₃→，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;
		}
    }