题解 | #单链表的排序# java归并排序

import java.util.*;


public class Solution {
    public ListNode sortInList (ListNode head) {
        // write code here
        if(head == null || head.next == null)
            return head;
        ListNode fast = head.next,p = head;
        while(fast != null && fast.next !=  null ){
            p = p.next;
            fast = fast.next.next;
        }
        ListNode lt = p.next;
        p.next = null;
        ListNode l1 = sortInList(head);
        ListNode l2 = sortInList(lt);
        return merge(l1,l2);
    }
    ListNode merge(ListNode l1 , ListNode l2){
        if(l1 == null){
            return l2;
        }
        if(l2 == null){
            return l1;
        }
        ListNode head;
        if(l1.val <= l2.val){
            head = l1;
            l1 = l1.next;
        }
        else{
            head = l2;
            l2 = l2.next;
        }
        ListNode p = head;
        while(l1 != null && l2 != null){
            if(l1.val < l2.val){
                p.next = l1;
                l1 = l1.next;
            }else{
                p.next = l2;
                l2 = l2.next;
            }
            p = p.next;
        }
        if(l1 != null){
            p.next = l1;
        }
        if(l2 != null){
            p.next = l2;
        }
        return head;
        }

}