/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode sortList(ListNode head) {
        //边界判断
        if(head == null ||head.next == null){
            return head;
        }
        //建立快指针和慢指针，快指针到头，慢指针到一半
        ListNode fast = head.next;
        ListNode slow = head;
        while(fast != null && fast.next != null){
            slow = slow.next;
            fast = fast.next.next;
        }
        //temp指针是分层两半指针的头指针
        ListNode temp = slow.next;
        slow.next = null;
        ListNode left = sortList(head);
        ListNode right = sortList(temp);
        ListNode h  = new ListNode(0);
        ListNode res = h;

        while(left != null && right != null){
            if(left.val < right.val){
                h.next = left;
                left = left.next;
            }else{
                h.next = right;
                right = right.next;
            }
            h = h.next;
        }
        h.next = left!= null?left:right;
        return res.next;
    }
}

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