leetcode_Sort List

 public ListNode sortList(ListNode head)
	{
		if(head==null||head.next==null)
			return head;
		ListNode midNode=getMidNode(head);
		ListNode newHead=midNode.next;
		midNode.next=null;
		head=mergeLists(sortList(newHead), sortList(head));
		return head;
	}

	public ListNode mergeLists(ListNode head1, ListNode head2)
	{
		if(head1==null&&head2==null)
			return head1;
		if(head1==null)
			return head2;
		if(head2==null)
			return head1;
		ListNode newHead=new ListNode(-1);
		ListNode curListNode=newHead;
		while(head1!=null&&head2!=null)
		{
			if(head1.val<head2.val)
			{
				curListNode.next=head1;
				head1=head1.next;
			}else 
			{
				curListNode.next=head2;
				head2=head2.next;
			}
			curListNode=curListNode.next;
		}
		if(head1!=null)curListNode.next=head1;
		if(head2!=null)curListNode.next=head2;
		return newHead.next;
	}

	public ListNode getMidNode(ListNode head)
	{
		if(head==null||head.next==null)
			return head;
		ListNode p=head,q=head;
		while(q.next!=null&&q.next.next!=null)
		{
			p=p.next;
			q=q.next.next;
			
		}
		return p;
	}

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