单向链表的归并排序（Java）

 1 package SortList;
 2 
 3 import java.util.Iterator;
 4 
 5 class ListNode {
 6 
 7     int val;
 8     ListNode next;
 9     ListNode(int val) {
10         this.val = val;
11     }
12 }
13 
14 public class Solution {
15     public static ListNode sortList(ListNode head) {
16 
17         if (head == null || head.next == null) {
18             return head;
19         }
20         /**
21          * 定义两个指针，其中一个是另一个的二倍，
22          * 等前一个到达链表结尾时，后一个到达链表中间
23          */
24         ListNode before = head.next;
25         ListNode after = head;
26         while (before.next != null && before.next.next != null) {
27             before = before.next.next;
28             after = after.next;
29         }
30 
31         ListNode l2 = sortList(after.next);
32         after.next = null;
33         ListNode l1 = sortList(head);
34 
35         return merge(l1, l2);
36 
37     }
38 
39     static ListNode merge(ListNode list1, ListNode list2) {
40 
41         /**
42          * 先初始化一个空链表
43          * 并赋予list1和list2中的一个节点
44          */
45         ListNode list = null;
46         if(list1.val <= list2.val){
47             list = list1;
48             list1 = list1.next;
49         } else {
50             list = list2;
51             list2 = list2.next;
52         }
53         ListNode lis = list;
54         while (list1 != null && list2 != null) {
55             if (list1.val <= list2.val) {
56                 lis.next = list1;
57                 list1 = list1.next;
58                 lis = lis.next;
59             } else {
60                 lis.next = list2;
61                 list2 = list2.next;
62                 lis = lis.next;
63             }
64         }
65         if (list1 == null) {
66             lis.next = list2;
67         } else {
68             lis.next = list1;
69         }
70         return list;
71     }
72 
73     public static void main(String[] args) {
74         ListNode list = new ListNode(73);
75         ListNode list1 = new ListNode(11);
76         ListNode list2 = new ListNode(25);
77         ListNode list3 = new ListNode(34);
78         ListNode list4 = new ListNode(18);
79         list.next = list1;
80         list1.next = list2;
81         list2.next = list3;
82         list3.next = list4;
83         ListNode llll = sortList(list);
84         //System.out.println(llll.next.next.val);
85         while (llll != null) {
86             System.out.println(llll.val);
87             llll = llll.next;
88         }
89         // System.out.println(llll.toString());
90     }
91 
92 }