在 O(n log n) 時間復雜度和常數級空間復雜度下，對鏈表進行排序。

示例 1:

輸入: 4->2->1->3
輸出: 1->2->3->4

示例 2:

輸入: -1->5->3->4->0
輸出: -1->0->3->4->5

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 
 
*/
class Solution {
    public ListNode sortList(ListNode head) {
        if(head == null || head.next == null) return head;
        
        ListNode fast = head, slow = head, pre = slow;
        while(fast != null && fast.next != null) {
            pre = slow;
            slow = slow.next;
            fast 
 
= fast.next.next;
        }
        pre.next = null;
        
        ListNode l = sortList(head);
        ListNode r = sortList(slow);
        
        return merge(l, r);
    }
    
    private ListNode merge(ListNode l, ListNode r) {
        ListNode q = new ListNode(0), p = q;
        
 
while(l != null && r != null) {
            if(l.val < r.val) {
                p.next = l;
                l = l.next;
            } else {
                p.next = r;
                r = r.next;
            }
            p = p.next;
        }
        
        if(l != null) p.next = l;
        if(r != null) p.next = r;
        return q.next;
    }
}

力扣——排序鏈表