和数组里面的归并排序思想一致

class Solution {
    public ListNode sortList(ListNode head) {
        //过滤条件
        if(head == null || head.next == null)return head;
        ListNode slow = head;
        ListNode fast = head.next;
    
        while (fast != null && fast.next != null){
            slow = slow.next;
            fast = fast.next.next;
        }
        ListNode rightHead = slow.next;
        slow.next = null;
        ListNode left = sortList(head);
        ListNode right = sortList(rightHead);
        
        return mergeList(left, right);
    }
    public ListNode mergeList(ListNode l1, ListNode l2){
        if(l1 == null)return l2;
        if(l2 == null)return l1;
        ListNode dummyNode = new ListNode(-1);
        ListNode curr = dummyNode;
        while (l1 != null && l2 != null){
            if(l1.val <= l2.val){
                curr.next = l1;
                l1 = l1.next;
            }else {
                curr.next = l2;
                l2 = l2.next; 
            }
            curr = curr.next;
        }
        if(l1 == null)curr.next = l2;
        if(l2 == null)curr.next = l1;
        return dummyNode.next;
    }

    }