reorder-list

/**

 * Definition for singly-linked list.

 * class ListNode {

 *     int val;

 *     ListNode next;

 *     ListNode(int x) {

 *         val = x;

 *         next = null;

 *     }

 * }

 */

//思路:通过不同的首结点获取到不同的尾结点,然后拼接

public class Solution {

   public ListNode getLastNode(ListNode head)

    {

        ListNode first = head;

        ListNode fakeLast = head;

        if (head.next == null)

        {

            return head;

        }

        while (first.next != null)

        {

            fakeLast = first;

            first = first.next;

        }

        ListNode last = fakeLast.next;

        fakeLast.next = null;

        return last;

    }

    public void reorderList(ListNode head)

    {

       if(head==null){

            return;

        }

        // 真的首结点

        ListNode first = head;

        // 假的首结点

        ListNode fakeFirst = head;

        while (first.next != null)

        {

            ListNode last = getLastNode(first);

            fakeFirst = first.next;

            if (fakeFirst == null)

            {

                first.next = last;

                break;

            }

            else

            {

                first.next = last;

                last.next = fakeFirst;

                first = fakeFirst;

            }

        }

    }

}