LeetCode解题报告:Reorder List

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}.

思路：

1.利用快慢两个指针将链表一分为二；

2.针对第二个子链表求倒序；

3.利用merge思想将两个子链表合并。

代码：

 public class ReorderSingleList {
     /**
      * Definition for singly-linked list. class ListNode { int val; ListNode
      * next; ListNode(int x) { val = x; next = null; } }
      */
     public void reorderList(ListNode head) {
         if (head == null || (head.next == null)) {
             return;
         }
         // fast and slow point find the mid position.
         ListNode fast = head;
         ListNode slow = head;
         while ((fast != null) && (fast.next != null)) {
             fast = fast.next.next;
             slow = slow.next;
         }

         // reverse the last second list.
         ListNode headnode = new ListNode(-1);
         headnode.next = slow;
         ListNode temp = headnode.next;
         while (temp.next != null) {
             ListNode insert = temp.next;
             ListNode currNext = insert.next;
             insert.next = headnode.next;
             headnode.next = insert;
             temp.next = currNext;
         }

         // merge insert
         ListNode firstcur = head;
         ListNode secondcur = headnode.next;
         while (firstcur != slow && (secondcur != slow)) {// at first,I make a mistake in here;
             ListNode firstnex = firstcur.next;
             ListNode secondnex = secondcur.next;
             firstcur.next = secondcur;
             secondcur.next = firstnex;
             firstcur = firstnex;
             secondcur = secondnex;
         }
     }

     public static void main(String[] args) {
         ListNode t5 = new ListNode(5);
         ListNode t4 = new ListNode(4, t5);
         ListNode t3 = new ListNode(3, t4);
         ListNode t2 = new ListNode(2, t3);
         ListNode t1 = new ListNode(1, t2);
         new ReorderSingleList().reorderList(t1);
         System.out.println(t1);
     }

 }