Sort List ——LeetCode

Sort a linked list in O(n log n) time using constant space complexity.

链表排序，要求时间复杂度O(nlgn)，我写的归并排序。

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode sortList(ListNode head) {
        if(head==null){
            return head;
        }
        ListNode  tail = head;
        while(tail.next!=null){
            tail=tail.next;
        }
        return mergeList(head,tail);
    }

    ListNode mergeList(ListNode head,ListNode tail){
        if(head==tail){
            return head;
        }
        ListNode mid = getMid(head,tail);
        ListNode postList = mergeList(mid.next,tail);
        mid.next=null;
        ListNode preList = mergeList(head,mid);
        return merge(preList,postList);
    }

    ListNode getMid(ListNode head,ListNode tail){
        if(head==tail){
            return head;
        }
        ListNode fast = head,slow = head;
        while(fast.next!=null&&fast.next.next!=null&&fast.next!=tail){
            fast=fast.next.next;
            slow=slow.next;
        }
        return slow;
    }
    ListNode merge(ListNode l1,ListNode l2){
        if(l1==null||l2==null){
            return l1==null?l2:l1;
        }
        ListNode ptr = new ListNode(0);
        ListNode head = ptr;
        while(l1!=null&&l2!=null){
            if(l1.val<=l2.val){
                ptr.next = l1;
                l1=l1.next;
            }else{
                ptr.next = l2;
                l2=l2.next;
            }
            ptr=ptr.next;
        }
        ptr.next=l1==null?l2:l1;
        return head.next;
    }
}