leetcode Sort Listnode

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

findMiddle: 将listnode 不停的拆分

sort： 将拆分好的不同的sort

merge： 将sort好的，向上merge

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode sortList(ListNode head) {
      if(head==null||head.next==null){
        	return head;
        }
        ListNode mid=findMiddle(head);
        
        ListNode right=sortList(mid.next);
        mid.next=null;
        ListNode left=sortList(head);
        return merge(left,right);
    }
    private ListNode findMiddle(ListNode head){
    	ListNode slow=head;
    	ListNode fast=head.next;
    	while(fast!=null&&fast.next!=null){
    		fast=fast.next.next;
    		slow=slow.next;
    	}
    	return slow;
    }
    private ListNode merge(ListNode head1,ListNode head2){
    	ListNode result=new ListNode(0);
    	ListNode temp=result;
    	while(head1!=null&&head2!=null){
    		if(head1.val<=head2.val){
    			temp.next=head1;
    			head1=head1.next;
    		}
    		else{
    			temp.next=head2;
    			head2=head2.next;
    		}
    		temp=temp.next;
    	}
    	if(head1!=null){
    		temp.next=head1;
    	}
    	else{
    		temp.next=head2;
    	}
    	return result.next;
    }
}