排序链表

在 O(n log n) 时间复杂度和常数级空间复杂度下，对链表进行排序。

示例 1:

输入: 4->2->1->3
输出: 1->2->3->4

示例 2:

输入: -1->5->3->4->0
输出: -1->0->3->4->5

使用归并排序

/**
 * 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 oHead = head;
        ListNode pre=null,quick=head,slow=head;
        while(quick!=null){
            quick = quick.next;
            if(quick==null){
                break;
            }
            pre = slow;
            slow = slow.next;
            quick = quick.next;
        }
        if(pre!=null){
          pre.next = null;
        }
        ListNode left = sortList(oHead);
        ListNode right = sortList(slow);
        ListNode result = mergeSort(left,right);
        return result;
    }
    //俩链表合并
    private ListNode mergeSort(ListNode head1,ListNode head2){
        if(head1==null){
            return head2;
        }
        if(head2==null){
            return head1;
        }
        ListNode head=null,node=null;
        while(head1!=null && head2!=null){
            ListNode now=null;
            if(head1.val < head2.val){
                now = head1;
                head1 = head1.next;
            }else{
                now = head2;
                head2 = head2.next;
            }
            if(head == null){
                head = now;
                node = now;
            }else{
                node.next = now;
                node=now;
            }
        }
        while(head1!=null){
            node.next = head1;
            node = head1;
            head1=head1.next;
        }
        while(head2!=null){
            node.next = head2;
            node = head2;
            head2 = head2.next;
        }
        return head;
    }
}