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}.

思路：

1、将链表从中间分成两部分；

2、将后面部分链表反转；

3、将两个链表合起来；

AC代码：

public class ReorderList {public void reorderList(ListNode head){if( head == null || head.next == null ){return;}//partition the list into two sublist of equal length//sublist1 is equal or more 1 than sublist2ListNode slowNode = head;ListNode fastNode = head;while( fastNode.next != null && fastNode.next.next != null ){slowNode = slowNode.next;fastNode = fastNode.next.next;}//sublist headListNode head1 = head;ListNode head2 = slowNode.next;//detach the list into two sublistslowNode.next = null;//2 reverse the second sublist, reverse the list when list's length bigger than 1ListNode curr = head2;ListNode post = curr.next;curr.next = null;while( post != null ){ListNode temp = post.next;post.next = curr;curr = post;post = temp;}head2 = curr;//3 merge the two sublistListNode curr1 = head1;ListNode curr2 = head2;while( curr2 != null ){ListNode post1 = curr1.next;ListNode post2 = curr2.next;curr1.next = curr2;curr2.next = post1;curr1 = post1;curr2 = post2;}}public static void main(String[] args) {// TODO Auto-generated method stub}}

测试用例：

import static org.junit.Assert.*;import junit.framework.Assert;import org.junit.After;import org.junit.Before;import org.junit.Test;public class ReorderListTest {private ReorderList reorder;@Beforepublic void setUp() throws Exception {}@Afterpublic void tearDown() throws Exception {}@Testpublic void testReorderList() {reorder = new ReorderList();ListNode list1 = new ListNode(1); ListNode list2 = new ListNode(2);ListNode list3 = new ListNode(3);ListNode list4 = new ListNode(4);list1.next = list2;list2.next = list3;list3.next = list4;list4.next = null;ReorderList reorder = new ReorderList();reorder.reorderList(list1);assertEquals(1, list1.val);assertEquals(4, list1.next.val);assertEquals(2, list1.next.next.val);assertEquals(3, list1.next.next.next.val);//System.out.println(list1.next.next.next.val);//fail("Not yet implemented");}}

未AC，因为使用额外空间（自己记录）：

//class ListNode{//int val;//ListNode next;//ListNode(int x){//val = x;//next = null;//}//}public class ReorderList {public void reorderList(ListNode head){if( head.next == null || head.next.next == null ){return;}ListNode dummyHead = new ListNode(-1);ListNode curr = dummyHead;ListNode preEnd = head;while( head.next != null && head.next.next != null ){while( preEnd.next.next != null ){preEnd = preEnd.next;}curr.next = head;head = head.next;curr = curr.next;curr.next = preEnd.next;curr = curr.next;preEnd.next = null;if( head.next == null || head.next.next == null){curr.next = head;break;}}head = dummyHead.next;}public static void main(String[] args) {// TODO Auto-generated method stubListNode list1 = new ListNode(4); ListNode list2 = new ListNode(1);ListNode list3 = new ListNode(3);list1.next = list2;list2.next = list3;list3.next = null;ListNode head = list1;ReorderList reorder = new ReorderList();reorder.reorderList(head);System.out.println(head.next.val);}}