leetcode 148. Sort List

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

/**
 * 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 || head.next==null){
          return head;
        }
        ListNode midd=getMidder(head);
        ListNode headR=midd.next;
        midd.next=null;
        return megger(sortList(head),sortList(headR));
    }
    private ListNode getMidder(ListNode head){
        if(head==null)
            return null;
        ListNode slow=head;
        ListNode fast=head;
        while(fast.next!=null && fast.next.next!=null){//这里不是fast!=null && fast.next!=null  !!!! 如 1-》2=》null
            slow=slow.next;
            fast=fast.next.next;
        }
        return slow;
    }
    private ListNode megger(ListNode headL,ListNode headR){
        if(headL==null)
            return headR;
        if(headR==null)
            return headL;
        ListNode tmpHead=new ListNode(-1);
        ListNode tmpNext=tmpHead;
        while(headL!=null && headR!=null){
            if(headL.val>=headR.val){
                tmpNext.next=headR;
                headR=headR.next;
            }
            else{
                tmpNext.next=headL;
                headL=headL.next;
            }
            tmpNext=tmpNext.next;
            tmpNext.next=null;
        }
        tmpNext.next = headL!=null?headL:headR; //这个不能少！！！
        return tmpHead.next;
    }
}

这篇文字说快排也可以，但没有测试过： http://blog.youkuaiyun.com/jiadebin890724/article/details/21334059