https://leetcode.cn/problems/sort-list/description/https://leetcode.cn/problems/sort-list/description/

解题思路：

归并排序，先拿到链表长度，每次遍历到一半，进行分割，后序双指针合并。

/**
 * 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 {
    ListNode gb(ListNode head, int len){
        if(len<=1){
            return head;
        }
        ListNode m_node=head;
        for(int i=1;i<len/2;i++){
            m_node=m_node.next;
        }
        ListNode t=m_node;
        m_node=m_node.next;
        t.next=null;
        ListNode now=new ListNode(),ans=now;
        ListNode l=gb(head,len/2);
        ListNode r=gb(m_node,len-len/2);
        int llen=len/2,rlen=len-len/2;
        while(llen!=0||rlen!=0){
            if(llen==0||(rlen>0&&l.val>=r.val)){
                ans.next=r;
                r=r.next;
                ans=ans.next;
                rlen-=1;
            }
            else{
                ans.next=l;
                l=l.next;
                ans=ans.next;
                llen-=1;
            }
        }
        return now.next;
    }
    public ListNode sortList(ListNode head) {
        int len=0;
        ListNode temp=head;
        while(temp!=null){
            temp=temp.next;
            len+=1;
        }
        return gb(head,len);
    }
}