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思路源自

【力扣hot100】【LeetCode 148】排序链表｜归并排序的应用

仿照归并排序 

先使用一个快慢指针将链表一分为二，然后进行排序归并

/*** Definition for singly-linked list.* public class ListNode {*     int val;*     ListNode next;*     ListNode() {}*     ListNode(int val) { this.val = val; }*     ListNode(int val, ListNode next) { this.val = val; this.next = next; }* }*/
class Solution {public ListNode sortList(ListNode head) {if (head == null || head.next == null) {return head;} else {ListNode slow = head, fast = head.next;while (fast != null && fast.next != null) {slow = slow.next;fast = fast.next.next;}ListNode secondPart = slow.next;slow.next = null;ListNode firstPart = head;ListNode sortFirPart = sortList(firstPart);ListNode sortSecPart = sortList(secondPart);return sortList(sortFirPart, sortSecPart);}}private ListNode sortList(ListNode l1, ListNode l2) {ListNode dymmyHead = new ListNode(0, null);ListNode cur=dymmyHead;while (l1 != null && l2 != null) {if (l1.val <= l2.val) {cur.next=l1;cur=cur.next;l1=l1.next;} else {cur.next=l2;cur=cur.next;l2=l2.next;}}while (l1 != null) {cur.next=l1;cur=cur.next;l1=l1.next;}while (l2 != null) {cur.next=l2;cur=cur.next;l2=l2.next;}return dymmyHead.next;}
}