LeetCode Reorder List

Given a singly linked list L: L0→L1→…→Ln-1→Ln,
reorder it to: L0→Ln→L1→Ln-1→L2→Ln-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}.

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public void reorderList(ListNode head) {
        if (head!=null) {
            LinkedList<ListNode> list = new LinkedList<>();
            ListNode temp=head;
    //        list.addLast(temp);
            boolean flag=false;
            while (temp.next!=null) {
                list.addLast(temp.next);
                temp=temp.next;
            }
            temp=head;
            while (!list.isEmpty()) {
                if (flag) {
                    temp.next=list.pollFirst();
                    flag=false;
                }else {
                    temp.next=list.pollLast();
                    flag=true;
                }
                temp=temp.next;
            }
            temp.next=null;
            
        }
    }
}